Main content

### Course: Class 8 Math (Assamese) > Unit 7

Lesson 1: Percent change word problems# Percent word problem: penguins

You're given the percent in this example and asked to find the whole number represented by the percent. Created by Sal Khan.

## Want to join the conversation?

- At1:27why didn't sal simplify his work to 3/15?(19 votes)
- Well, actually if you want to get 3 you have to do 0.3*10. But, you will also get 150 by doing 15*10. As Sal said, you can't multiply only one number (or decimal).
.**Sorry no one answered before**; but it's a good question(7 votes)

- could you just times 15 by 3 then find how much 10 percent (the remaining percent after multiplying 15 by 3, witch will = 90%) of 15 is?(5 votes)
- Seems like you are doing: 30% of 15, which is a different problem to the one in the video.

The problem in the video tells you that 30% of some number = 15

You need to divide by 0.3 not multiply.

Hope this helps.(6 votes)

- Another problem I got was: "81 is 56% of what?" I got 144.6429... and it kept going on. Can I just keep it at 144.6?

Thanks!(5 votes)- Same comment as for your other question... see if you have instructions that tell you where to round the answer (what place value).(3 votes)

- At
why is00:44*30% of X*is same as 30%*x, why are we multiplying instead of adding?(5 votes)- "of" basically means "*". like 0.5 of 20 is 0.5 * 20 It makes sense in decimals.(3 votes)

- how about if you are trying to find the percent not how many penguins live in zoo?(2 votes)
- Construct a different equation to solve for the unknown variable. In the above video you would have something like:

50X = 15

Solve for X gives you 0.3 which = 30%(5 votes)

- Is there an easy way to work out percentage increase and reduction? My mum showed me an exceedingly complicated way and I don't understand it!(0 votes)
- Percentages are just fractions where the bottom part is always
`/100`

. So`30%`

is the fraction`30/100`

. If you multiply or divide by a percentage, you're just multiplying or dividing by that fraction.(7 votes)

- is there a way to make this video simpler because it's not really helping me, just making me more confused.(3 votes)
- Another way to think about this is to solve a proportion of
`%/100 = is/of`

. The more you practice and substitute the values for`%`

,`is`

, and`of`

, then you can solve percents easily.(1 vote)

- need help ansering this question .The marching band has 100 members .Twelve of them play the flute .What percentage of the band members play the flute ?(2 votes)
- In a previously answered question,

"What is the sale price for a pair of shoes that originally cost $64 and are 25% off? HELP ME!" down below, the answer stated that it was 48%. I decided to try out the problem, for extra practice on percents. I made it into x/64 = 25/100, resulting in the answer 16. Just to make sure... would I have to subtract 16 from 64 because in the previous equation, I was mainly trying what 25% off of $64 was? Thank you!(2 votes)- I was taught how to do that to.My mom taught me how to do it becuse math was really hard for me.This way made it easier.I think that you should subtract 16 from 64 because the whhole is $64 and the part is $16.(1 vote)

- wow! this is kind of helpful! I'm studying for a test, and this clears things up! What is the difference between decimals and fractions?(2 votes)
- A decimal is basically a type of fraction, or a fraction is a type of decimal. They have the same "outcomes", except they are in different forms. For example, 80/100 is a fraction, the decimal would be 0.80. All decimals and fractions have as a difference are the arranging. Oh yeah, and most of the time a fraction is more simple than a decimal; you would write 0.3333333333333333..., not 3333333333333333/10000000000000000.(1 vote)

## Video transcript

A zoo has 15
emperor penguins who make up 30% of the total
number of penguins at the zoo. How many penguins
live at the zoo? So let's let x equal the
total number of penguins who live at the zoo. So they're telling
us that 30% of that are the emperor
penguins, which are 15. So they're saying if we
take 30%, so 30% of x is equal to 15. Or another way of saying
this is we could say, Instead of writing 30%, we
could write that as a decimal, as 0.30 of x. So it's 0.30 times
x is equal to 15. And now to solve this, we
just have to divide both sides by 0.30. So let's do that. 0.30, 0.30, we get x is
equal to 15 divided by 0.30. We're going to have to do a
little arithmetic to solve this one. So if we take 0-- it's
15 divided by 0.30. So just as a refresher
of dividing decimals, we could essentially multiply
both of these by 100. And when you multiply
both of them by 100, you can't just
multiply one of them. Then you would get
a different answer. But if you multiply
both of them by 100, you move this decimal point over
two spots, so it becomes a 30. And you move this decimal over
two spots, so it becomes 1,500. So it really boils down
to how many times-- the same number of times that
30 goes into 1,500 is the number of times that 0.30
will go into 15. So let's think about this. Let me rewrite it here,
just so it's neat. 30 goes into 1,500, all we did
is we moved both decimals over to the right twice. Or you could say
we just multiplied the numerator and
the denominator here by 100, which
wouldn't change the value of the fraction. So 30 goes into-- let's
see, it doesn't go into 1. It doesn't go into 15. It does go into 150. It goes into 150 five times. 5 times 30 is 150. Subtract, you get 0. And then 30 goes
into 0 zero times. So this 30 goes into
1,500 fifty times. So this right over
here is equal to 50. And you can verify it. Multiply 0.3 times 50,
and you will get 15. Now, there is another
way of doing it. You could say the fraction
of emperor penguins over the total number
of penguins at the zoo. So you could say 15. You could say 15 over the
total number of penguins at the zoo is equal to 30%. Percent literally
means 30 hundreds. So it's equal to
30 for every 100. It's equal to 30 for every 100. And then you could solve
for this right over here. And a really simple way
of doing this is you say, look, to go
from 30 to 15, you have to divide by 2 if
you're going from the right to the left. So let's divide by
2 right over here. So x would be equal to 50. 15/50 is the same
thing as 30/100. So the total number
of penguins are 50.