Main content

## Algebra (all content)

### Course: Algebra (all content) > Unit 2

Lesson 12: Old school equations with Sal# Taking percentages

Taking a percentage of a number. Created by Sal Khan.

## Want to join the conversation?

- How do you do problems like : 36 is what percent of 150?(4 votes)
- You divide 36 by 150, which is 0.24

Then, you take the answer and multiply it by 100%, which is really just multiplying by a 100 then putting a percent sign at the end.(6 votes)

- how do you know if you should multiply or divide?(4 votes)
- If you know what the whole number is and you know what percent of that number you are looking for, you multiply. For example, if you are looking for 20% of 100, you multiply 100 by 0.2. If you want to find what percent of 100 is equal to 20, you would divide 100 by 20.(3 votes)

- Wait, at2:10-2:47, I always thought that .2% also equals .20 as a percentage. Please Help!(1 vote)
- 100% in decimal form is unity, so .2% is .2/100=.002(4 votes)

- If this helps anyone. I always think that when there is a, "of" then that means divide the two numbers on the left and right of the of. , and "is" means = sign.. So 4=20%x what number. Hope that helps.(3 votes)
- Normally When it says of it means multiply actually all of the time but you can do the reciprocal of the number and then you can divide to get the same answer kind of like 1/2 you can reciprocal it to get to then divide by that number to get your answer(0 votes)

- what is 20 percent of 560(0 votes)
- 560 * .20 = 112(3 votes)

- What is the best way to explain problems like 7(.2) = 70(.02)?(2 votes)
- 7(0.2 = 7(2/10) = 14/10 Reduce by factor of 2: 7/5

70(0.02) = 70(2/100) = 140/100. Reduce by factor of 20: 7/5

They are the same value.

Hope this helps.(1 vote)

- how do I find the original price if given total price and tax rate(2 votes)
- Great question! Say the tax was 10% and the total was 132. Then the total price was 100% original price + 10% original price, so 110% original price. That's 1.1*original price. Divide 132 by 1.1 (see dividing decimals) and you get 120!(0 votes)

- wouldn't it be 0.02?

why is it 0.002?(1 vote) - 60% of what number is 45?. Please show me how to arrive at the answer. Thanks.(1 vote)
- If you want to do it "the right way", you have to divide 45 by 60 and then multiply the result by 100. If you want to make it easier, knowing that 60/100 is equal to 3/5, you can also divide 45 by 3 and then multiply the result by 5. The result is 75.(2 votes)

- In a population 3% of the people are born twins.What percentage of the population is twins?(1 vote)
- 3%?

Assuming twins have the same lifespan as non-twins.(2 votes)

## Video transcript

Let's get started
with some problems. Let's see. First problem: what
is 15% of 40? The way I do percent problems
is I just convert the percentage to a decimal and
then I multiply it times the number that I'm trying to
get the percentage of. So 15% as a decimal is 0.15. You learned that from the
percent to decimal conversion video, hopefully. And we just multiply
this times 40. So let's say 40 times 0.15. 5 times 0 is 0. 5 times 4 is 20. Put a 0 there. And then 1 times 0 is 0. 1 times 4 is 4. And you get 6 0 0. Then you count the
decimal spots. 1, 2. No decimals up there,
so you go 1, 2 and you put the decimal there. So 15% of 40 is equal to 0.15
times 40, which equals 6.00. Well, that's just the
same thing as 6. Let's do another problem. Hopefully, that didn't
confuse you too much. And I'm going to try to
confuse you this time just in case you weren't properly
confused the last time. What is 0.2% of-- let me
think of a number-- of 7. So a lot of people's
inclinations would just say, oh, 0.2%, that's
the same thing as 0.2. And if that was your
inclination you would be wrong. Because remember,
this isn't 0.2. This is 0.2%. So there's two ways of
thinking about this. You could say that this is
0.2/100, which is, if you multiply the numerator and
denominator by 10, is the same thing as 2/1,000. Or you can just do the
technique where you move the decimal space
over 2 to the left. In which case, if you're
starting with 0.2 and you move the decimal space 2
to the left, you go bam. Whoops! Bam, bam. That's where the decimal goes. So it's 0.002. This is key. 0.2% is the same
thing as 0.002. This can always trip you up and
I've made this careless mistake all the time, so don't feel
bad if you ever do it. But just always pay careful
attention if you see a decimal and a percentage
at the same time. So now that we've figured out
how to write this percentage as a decimal we just have to
multiply it times the number that we want to take
the percentage of. So we say 0.002 times 7. Well, this is pretty
straightforward. 7 times 2 is 14. And how many total numbers do
we have or how many total digits do we have behind
the decimal point? Let's see. It's 1, 2, 3. So we need 1, 2, 3 digits
behind the decimal point. So 0.2% of 7 is equal to 0.014. And you're probably thinking,
boy, that's a really, really small number. And it makes sense because
0.2%, if you want to think about it, that's
smaller than even 1%. So that's even
smaller than 1/100. And actually, if you think
about it, 0.2% is 1/500. And if you do the math,
1/500 of 7 will turn out to be this number. And that's an important
thing to do. It's always good to do a
reality check because when you're doing these decimal and
these percent problems, it's very easy to kind of lose a
factor of 10 here or there. Or gain a factor of 10. So always do a reality check to
see if your answer makes sense. So now I'm going to
confuse you even further. What if I were to ask you
4 is 20% of what number? So a lot of people's
reflex might just be, oh, let me take 20%. It becomes 0.20. And multiply it times 4. And in that case, again,
you may be wrong. Because think about it. I'm not saying
what is 20% of 4? I'm saying that 20%
of some number is 4. So now we're going to be doing
a little bit of algebra. I bet you didn't expect that
in the percent module. So let x equal the number. And this problem says that
20% of x is equal to 4. I think now it's in a form
that you might recognize. So how do we write
20% as a decimal? Well, that's just 0.20 or 0.2. And we just multiply
it by x to get 4. So 20%, that's the
same thing as 0.2. It's the same thing as 0.20,
but that last trailing 0 doesn't mean much. 0.2 times x is equal to 4. And now we have a level
one linear equation. I bet you didn't
expect to see that. So what do we do? Well there's two
ways to view it. You can just divide both
sides of this equation by the coefficient on x. So if you divide 0.2 here
and you divide by 0.2 here. So you get x is equal
to 4 divided by 0.2. So let's figure out what
4 divided by 0.2 is. I hope I have enough space. 0.2 goes into 4-- I'm going
to put a decimal point here. And the way we do these
problems, we move the decimal point here one
over to the right. So we just get a 2 and then we
can move the decimal point here one over to the right. So this 0.2 goes into 4
the same number of times that 2 goes into 40. And this is easy. 2 goes into 40 how many times? Well, 2 goes into 4 two
times and then 2 goes into 0, zero times. You could've done
that in your head. 2 into 40 is twenty times. So 4 divided by 0.2 is 20. So the answer is
4 is 20% of 20. And does that make sense? Well, there's a couple of
ways to think about it. 20% is exactly 1/5. And 4 times 5 is 20. That makes sense. If you're still not sure
we can check the problem. Let's take 20% of 20. So 20% of 20 is equal
to 0.2 times 20. And if you do the math
that also will equal 4. So you made sure you
got the right answer. Let's do another one like that. I'm picking numbers randomly. Let's say 3 is 9% of what? Once again, let's let x equal
the number that 3 is 9% of. You didn't have to
write all that. Well, in that case we know that
0.09x-- 0.09, that's the same thing as 9% of x--
is equal to 3. Or that x is equal to
3 divided by 0.09. Well, if we do the decimal
division, 0.09 goes into 3. Let's put a decimal point here. I don't know how many
0's I'm going to need. So if I move this decimal over
to the right twice, then I'll move this decimal over
to the right twice. So 0.09 goes into 3 the
same number of times that 9 goes into 300. So 9 goes into 30 three times. 3 times nine is 27. I think I see a
pattern here already. 30, 3, 3 times 9 is 27. You're going to keep getting
33-- the 3's are just going to go on forever. So it turns out that 3 is 9%
of-- you can either write it as 33.3 repeating or we all know
that 0.3 forever is the same thing as 1/3. So 3 is 9% of 33 and 1/3. Either one of those would
be an acceptable answer. And a lot of times when you're
doing percentages you're actually just trying
to get a ballpark. The precision might not always
be the most important thing, but in this case we
will be precise. And obviously, on tests
and things you need to be precise as well. Hopefully, I didn't go too
fast and you have a good sense of percentage. The important thing for these
type of problems is pay attention to how the
problem is written. If it says find 10% of 100. That's easy. You just convert 10% to a
decimal and multiply it by 100. But if I were to ask you
100 is 10% of what? You have to remember that
that's a different problem. In which case, 100 is 10% of--
and if you did the math, it would be 1,000. I think I spoke very quickly on
this problem on this module, so I hope you didn't
get too confused. But I will record more.