| 3 species of percent computation problems |Finding the component |Finding the percent | finding the entirety | including percents| | additional Applications | Calculator brief cuts | rapid quiz |

We have already looked at a usual application the percent in the section, an essential Ideas: percent increases and also decreases. These basic examples autumn into the category of problems below, however were introduced earlier due to the fact that they are troubles with which many of us room familiar.

3 types of percent computation difficulties

There space 3 basic types that percent computation problems which deserve to be represented using the following expression,

r% of b is p

where r is the percent, b is the entirety or base quantity and p is the part of b.

You are watching: What percent of 80 is 4

2% of 200 is 4 r% b p percent whole part

In each form of percent problem we desire to discover one that the following: - the percent (r) - the whole (b) - the component (p)

Using the dual-scale number line we can quickly see which item of info is missing in each form of problem. Please see an interpretation and Models, Percent models and an essential Ideas for an ext details

For example, 12 out of 60 books on the shelf are chef books. This means 20% of the publications on the shelf are chef books.
finding the part (p) finding the percent (r) finding the whole (b)


What is 20% that 60 books? 12 out of 60 books is what percent? 12 is 20% of what number?

Because that the connection of percent to typical fractions and also decimals there room a number of approaches we have the right to use as a starting point to settle percent problems:

Benchmark - Recognise usual percents, such as 50% and also then do the computation if necessary.
Fractions - to express the percent together a fraction, such as 54/100, and also then carry out the computation using portion algorithms if necessary.
Decimal - refer the percent as a decimal, such as 0.54, and also then perform the computation making use of decimal algorithms if necessary.
Unitary - discover 1% of the quantity and then execute computation if necessary.
Calculator - Use quick cut methods noted on part calculators

We often use the unitary or benchmark approach when functioning out percent troubles in our heads. We will certainly look at exactly how we use these methods to resolve each problem type below.

Finding the part

Example 1: What is 5% of 80 kilometres?

What is r% of b? We require to uncover p, the part.

There are many different methods in which we can solve this problem. If we offered either the the unitary or benchmark approaches described listed below we would certainly usually do them in our heads.

Estimating the price Before fixing a trouble it is a an excellent idea to do an calculation of our answer for this reason that we can examine it for reasonableness when we have actually completed the problem. In this instance we could note the 5% that 100 is 5 so us would mean our answer to be much less than 5.

Working Out Thinking

0.8 is 1% that 80

0.8 x 5 = 4

4 is 5% the 80

Unitary approach 80 is 100% that 80, therefore 0.8 is 1% of 80.

Now we have actually 1% that 80 we deserve to multiply through 5 to obtain 5%.

8 is 10% that 80

8 ÷ 2 = 4

4 is 5% that 80

Benchmark method I understand that 8 is 10% of 80, so to uncover 5% I divide 8 through 2.



4 is 5% of 80




p = 4

4 is 5% the 80

Fraction approach 1 We might express 5% as a portion out the 100 and simplify it right into the equivalent fraction 1/20. We understand that 1/20th of 80 is 4.

Fraction strategy 2 we could also say 5 the end of 100 equals p out of 80 due to the fact that we desire to uncover out what component of 80 equals 5%. We then carry out the very same to both political parties of the equation to find p.

5% of 80 = 0.05 x 80

4 is 5% the 80

Decimal approach We might express 5% as a decimal, 0.05, and then multiply by 80.

Using mine calculator,

Enter 80 push x go into 5 press %

Answer: 4

4 is 5% the 80

Calculator strategy We can use the shortcut method detailed on part calculators which have actually a "%" button.

NOTE: the way you get in the numbers and symbols will count on your calculator - girlfriend will need to experiment. Note likewise that you are not have to keying in a exactly mathematical sentence, yet rather making use of a shortcut method listed by her calculator.

Example 2: Movie - an easy example of detect the percentage of hard numbers utilizing a calculator. What is 12.3% of $1294?

click on the photo to view the movie

We have the right to summarise what we have been doing when using the decimal or portion approach into a formula.

In bespeak to discover the part we should multiply the totality amount by the percent. Example: What is 15% of $300? p= (15/100) x 300 p= 45 15% of $300 is $45

finding the percent

Example 3: What percent is $4 the $80?

What percent is ns of b? We need to find r, the percent.

Estimating our answer us will first estimate ours answer. 10% the $80 is $8 (10 x 8 = 80) so we recognize our answer will be less than 10%.

Working Out Thinking



4 is 5% the 80




4 is 5% the 80

Fraction method 1 Our last answer will be as a fraction with a denominator of 100. Just how do we obtain there?

We can simplify the fraction to a type which us can an ext easily transform into hundredths and also then express as a percent.

Fraction technique 2 we could likewise say 4 the end of 80 amounts to r the end of 100. Us then perform the exact same to both sides of the equation to find r.


4 is 5% the 80

Decimal method We have the right to express 4/80 together a decimal utilizing our calculator and then express as a percent.

0.8 is 1% that 80

4 is ?% the 80

4 ÷ 0.8 = 5

4 is 5% that 80.

Unitary approach 80 is 100% that 80, for this reason 0.8 is 1% the 80.

We understand that 0.8 is 1% of 80 and also we desire to understand what percent 4 is that 80. We might find the end how plenty of 0.8 "s there room in 4. Four divided by 0.8 is 5.

NOTE: This technique is not as suited to this problem because we still have to do a lot of working out to get our answer. We can see the all various other approaches gain to the prize quicker.

8 is 10% the 80

4 is 5% of 80

Benchmark method I understand that 8 is 10% that 80. 4 is fifty percent of 8, therefore 4 should be 5% the 80.

Using mine calculator,

Enter 4 push / get in 80 press x get in 100

4 / 80 x 100 = 5% 4 is 5% that 80


Enter 4 push / go into 80 press % provides 5 directly

Calculator approach 4 is how many percent that 80?

In the very first case we space not utilizing the "%" button however are multiplying 4/80 by 100 to provide us the percent.

We could use the faster way method noted on part calculators which have a "%" button.

Example 4: Movie - recognize the percent making use of the portion approach. 48 world out of 360 human being studied drink Brand A softdrink regularly. What percent is this?

click on the picture to watch the movie

Movie - verifying ours answer to example 4 making use of the calculator.

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finding the whole

Example 5: 4 is 5% of what number?

p is r% the what number? We require to discover b, the whole.

Estimating ours answer we will first estimate our answer. 4 is 1% of 400, therefore 4 will be 5% the a lot smaller number. We understand that ours answer will certainly be a lot much less than 400.

Setting Out Thinking

4 is 10% of 40

4 is 5% of (2 x 40)

4 is 5% that 80

Benchmark method 4 is 5% of what number. We understand 4 is 10% the 40 (4 x 10 = 40), so because that 4 must be 5% of two times 40.


4 x100 = 5 x b 5b = 400

So, 4 is 5% that 80

Fraction technique We can express the problem in terms of fractions. 4 divided by some number, b, is identical to 5/100 (5%). We can then do the very same to both political parties of the equation to discover b.

5% = 0.05



4 is 5% of 80

Decimal method We have the right to express 5% together a decimal fraction, 0.05. 4 split by b is indistinguishable to 0.05 (5%). We have the right to then carry out the exact same to both political parties of the equation to uncover b.

4 is 1% the 400

4 is 5% that (400 ÷ 5)

4 is 5% of 80

Unitary technique 4 is 5% that what number? We know 4 is 1% that 400, for this reason 4 must be 5% of a number i beg your pardon is 5 time smaller.

4 is 5% of b go into 4 press / (divide) go into 5 press % provides 80 directly

Calculator approach There is a faster way for this trouble using my calculator.

Example 6: Movie - finding the entirety amount. Con noticed the the GST he payment on his TV was $36. Currently, the GST is 10% the the pre-GST price. Exactly how much walk the TV cost excluding GST? exactly how much walk it price including GST?

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Example 7: movies of a worded problem, finding the percent, percentage and the whole.

Movie A: detect the percent. Buy it shot 7 points out of she team"s full of 29 points at the basketball match last Saturday, what percent of points walk she shoot?

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Movie B: detect the part. Mary shot 17% that points, how numerous points walk she shoot?

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Movie C: finding the totality - estimating ours answer. If, in one more game, Mary and Sarah shooting 4 and also 6 purposes respectively, which amounted to 27% of your team"s total score, what was their team"s complete score?

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Movie D: recognize the entirety - calculating ours answer

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Adding percents

Example 8: If 30% of grade 10 students pertained to school through bus and 40% of great 10 students come to school by train, what percent that students pertained to school by public transport? (NOTE: Assume just one type of deliver is offered by every student)

Here i am talking about one team of students - class 10 students. I recognize 30% of all students involved school through bus and 40% the all students pertained to school by train. Both these develops of transport room public transport, so 40% + 30% = 70% the all students come to school by publicly transport.

Example 9: If great A and B each contain the same number of students and also 30% of student in class A room boys and also 40% of college student in course B space boys, what percent of class A and B linked are boys?

Here there space 2 equal teams of students, course A and also Class B. I understand that 30% of college student in class A are boys and 40% of students in course B. If both classes had actually 100 students, there would be 30 + 40 boys altogether the end of the total of 200 (= 100 + 100) students.

70 out of 200 is the exact same as 35 the end of 100 (because 70/200 = 35/100 are identical fractions).

So we deserve to say that while 30% of college student in class A room boys and 40% of student in class B room boys, 35% of class A and B linked are boys.

In the very first example, example 8, we space referring to one team of students so we can add the percents of students utilizing the bus and train to get to institution to gain the percent that students utilizing public transport.

In the second example, instance 9, we room talking around two equal groups of students.

NOTE: A common mistake is to include percents for teams which aren"t equal. This is no correct. When we want to find the linked percent of much more than one group, whereby the teams are of various size, we should take right into account the proportions that the groups.

Example 10: If 30% of student in course A room boys and 40% of student in class B room boys, and also Class A has actually 30 student in total and Class B has actually 20 students, what percent of class A and B combined are boys?

Here course A and also Class B perform not have actually the same variety of students. I have to convert the percentages come numbers, add the numbers and also then convert ago to a percent. In this case Class A has actually 9 boys and also Class B has 8 boys. Altogether, there room 17 boys the end of a total of 50 pupils. This is 34% that the an unified classes.

further Applications

Making comparisons

Although most human being understand the concept of percent, they still get perplexed when working through more complicated percent problems, an especially those which ask united state to compare the percents of different quantities. The following examples work with these species of problems.

Example 11: comparing percents. I have a 2 litre carton the milk the Brand A i beg your pardon is 95% fat complimentary and 1 litre carton of Brand B i m sorry is 98% fat free, if i pour one glass of each, i m sorry glass the milk contains much more fat?


Although the 2 litre carton that milk contains much more milk 보다 the 1 litre carton the fat cost-free percent is 95%. This way that in the 2 litre carton there space 95 components out that 100 components that do not save fat (or 5 parts out of every 100 which perform contain fat). The 1 litre carton is 98% fat free. This way that in this carton 98 components out the 100 parts do no contain fat (or 2 components out of every 100 carry out contain fat).

If we room comparing 2 glasses the milk, the glass containing Brand A milk will have 95 components out the 100 fat complimentary and the glass comprise Brand B milk will have actually 98 components out the 100 fat free. Therefore the glass of Brand A milk contains more fat.

Example 12: detect the percent of merged quantities.

Lee has 2 various sized packets the coloured jelly beans: one is 100g and the various other 250g. Each packet has 50g of black color jelly beans. Lee renders up a bowl v both packets the jelly beans. What is the percent of black jelly beans in the bowl?

Seeing together the packets have been mixed in the key we are interested in the percent of both packets together

- we understand that both packets have 50g of black jelly beans, so an unified there is 100g of black jelly beans - we understand that the complete weight that the packets is 100g + 250g = 350g - therefore we have actually 350 g that jelly beans in the bowl, 100 g of which room black jelly beans

To discover the percent of black jelly bean in the bowl we must ask, what percent is 100g the end of 350g?

100/350 = r/100 r/100 = 100/350 r = 100/350 x 100 r= 28.57 % (rounded)

Because we have various proportions of black color jelly beans in different sized packets, when combined the total number of black jelly beans is about 29%.

This explanation can make this difficulty sound fairly straightforward, however a usual error is to uncover the percent of black color jelly bean in every packet and also then include them. If we think about this the does not make sense. If we start off v (50/100) 50% of black color jelly bean in a tiny packet and also (50/250) 20% of black color jelly beans in a huge packet and also then we include our answers, we obtain the percent of black jelly bean in the bowl together 70%. This way we have a higher percent of black color jelly bean in the bowl than we had actually in one of two people packet to begin with! we cannot have an ext black jelly bean in the bowl than we started with.

NOTE: In troubles like this whereby we are talking around combining quantities and detect percents we should look in ~ the total quantities; in this case the total number of black jelly beans and also the total number of jelly beans.

Calculator brief cuts

Simple electronic calculators through a percent sign deserve to be really handy for including percents. This calculators usually market a shortcut method.

Example 13: utilizing a calculator to discover 110% of 57

Working Out Thinking

Using my calculator, go into 57 press x go into 110 press % Answer: 62.7

To discover 110% of 57 ns can crucial in

57 x 110 %

Note that the % vital tells the calculator to occupational out an answer: that is analogous to the = key.

The much more elementary the calculator the an ext likely a shortcut method is offered, however it will vary from calculator to calculator. Clinical calculators might not have actually this shortcut. You will notification that they usually perform not have actually a "%" sign and also you need to work out the difficulty by building a ideal mathematical sentence.

NOTE: The an approach will count on the calculator - friend will must experiment!

Example 14: using a calculator to add 5% that 30 to 7% of 45

Working Out Thinking

Using my calculator, enter 30 push x get in 5 press % come calculate press M+ to include to memory press CE to clear screen Enter 45 press x go into 7 push % to calculate press M+ to include to memory press MR to read answer Answer: 4.65

5% that 30 ns can key in together 30 x 5 %. I have the right to then usage the memory function to keep this answer 7% that 45 ns can crucial in together 45 x 7 %. I have the right to then include this to the memory and recall the last memory to view the answer.

(NOTE: If you are not sure how to usage the memory function of a calculator, check out the section whole Numbers, Multiplication, utilizing a calculator.)

Example 15: using a calculator to rise $120 by 7.2%

Working Out Thinking

Enter 120 press + enter 7.2 push % Answer: $128.64

This is a special quick cut for including a percent of a number. Check that it works on your calculator.

Example 16: making use of a calculator to take it a 15% discount indigenous $850.

Working Out Thinking

Using my calculator, enter 850 press - get in 15 push % Answer: $722.50

Again this is a one-of-a-kind shortcut. Check that it functions on her calculator before you count on it.

Quick quiz

1. A lengthy distance bus which travel from Melbourne come Sydney has 45 passenger seats.
(a) If 80% the the seats have been booked, how plenty of seats is this?
(b) If only 19 seats have actually been booked, what percent of seats is this?

There space 13 girls in Sari"s class which make up 45% that the totality class. How many students are in Sari"s class?


In one exam, Georgie reply 35 the end of 40 concerns correctly.

(a) What percent of the concerns did she prize correctly?
(b) If the pass note is 85%, go Georgie pass?
(c) If Georgie resat the test and also now scored 97%, how numerous questions go she answer correctly?
4. If a puppy put on 300 grams end the critical week and this is 15% of its total body weight, what go the puppy sweet now?

Ryan bought 15 concert tickets to market to his friends.

(a) So far he has actually sold 5 tickets to his friends, what percent has actually he sold?
(b) Ryan significant up the price of every ticket through 10% come pay because that his own ticket. If he payment $65 because that the tickets originally, just how much is he offering them because that now?
(c) Ryan has actually not had actually much success at selling the tickets and also now needs to discount them to get rid of them. If he discounts the price the $65 through 20%, exactly how much do the tickets cost now?
(d) at some point Ryan regulated to offer 73% of the 15 tickets, how plenty of did the sell?
6. Vinh to buy a pair of jeans and also a T-shirt that were on sale at a 25% discount. The original price the the jeans was $79 and the T-shirt was $35. Just how much walk Vinh pay because that each item?
7. In a city office of 35 workers, 14 catch public transfer to work, 8 ride their bikes, 10 drive your cars and also 3 walk.
(a) What percent of workers ride your bikes to work?
(b) What percent catch public transport?
(c) What percent drive or walk?
(d) What percent usage any an approach other than automobile to obtain to work?
(e) If these numbers were provided as the basis of a deliver survey exactly how many human being out the 100 could be stated to usage each technique of move to get to work?

See more: Solved Which Of The Following Describes Work Done, Which Of The Following Describes Work

One brand the pumpkin soup weighs 440 gs and also contains 35% fat. Another brand weighs 1 kg and also contains 20% fat. If I have actually a bowl of each, which bowl of soup contains an ext fat?

To see the quiz answers, click here.