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## Grade 7 (Virginia)

### Course: Grade 7 (Virginia) > Unit 4

Lesson 6: Finding percents# Finding common percentages

Find common percentages of a number by taking a familiar fraction of the number. Created by Sal Khan.

## Want to join the conversation?

- do it in my head? I'm okay-smart, but not that smart(4 votes)
- As a long time number sense supporter, I disagree with your statement, you are plenty smart to do a lot of things in your head, you just have not practiced it enough to get good at it.

If you look at each of the original problems, I believe the first two x1% and x10% would be easy for you, just move decimal two or one place to right. 20% = 10*2, so I believe you could double 45 and move decimal twice. For 25%, you could learn to divide 28 by 4 (since 25%=1/4). For 50%, you could divide 128 by 2 (50%=1/2). 400% = 4, so you could multiply 4*8. I am more confident than you that you can in fact you are plenty smart to do it in your head, all you need is a little number sense training.(13 votes)

- me do not get it(7 votes)
`You are just turning the percentage to a fraction, then simplifying it, then multiplying it by ___/1, and simplifying that.`

For some reason, I wrote this is in the code font.(4 votes)

- The Waffle House has found its new host.(4 votes)
- This is very fun and interesting when you understand it(4 votes)
- why does it have non equivalent percents?(3 votes)
- wow i was paying so much attention to the video(3 votes)
- I do not understand. Are u basically just doing something over something. Like here:

"What is 1% of 900"*This is Sal's question!!*So to solve this do u do:

1/900? Do you always have to simplify?

Thank you(3 votes) - What is 1% of 900 (if 900 was split into 100 pieces how much would each piece be worth?)

what is 10% of 650 ( If 650nwas split into 10 pieces how much would each piece be worth?)

What is 20% of 45 ( If 45 was split into 5 pieces how much would each piece be worth?)

What is 25% of 28 ( if 28 was split into 4 pieces how much would each piece be worth?)

What is 50% of 128 ( if 128 was split into 2 pieces how much would each p[iece be worth?)

What is 400% of 8 ( If you had 100% of 8 you would have 8, so how much would you have if increased that amount by 4 times)

Thats how i think about it, hope that helps. :)(2 votes)

## Video transcript

- [Instructor] What I would like you to do is pause this video and
see if you can calculate each of these percentages, and
ideally do it in your head. All right, now let's do it together. Now I said, how are you
going to do it in your head? You might be tempted to write down these as multiplication problems
and have to write it down. And that might be a reasonable thing, but these are particular percentages that you might see a lot of in life and so it's useful to think
about them in your head. For example, 1%. 1% is the same thing as 1 over 100. So 1% of 900 is the same thing as 1/100 of 900. And so this question boils down to really what is 900 divided by 100. And that of course is equal to 9. Let's do another example.
What's 10% of 630? Well 10% is equal to 10 over 100, which is the same thing as 1 over 10. So if I were to say 10% of 630, that's the same thing
as saying 1/10 of 630. So this all boils down
to 630 divided by 10, which you would recognize as 63. All right, let's do this
next one, 20% of 45. You might recognize already,
and if you haven't already, it's good to recognize that 20% is the same thing as 20 over 100, or that it's the same thing as 1 over 5. It's good to just know
that hey, 20% is 1/5. So if I'm saying 1/5 of 45, that's the same thing as 45 divided by 5, which is of course, equal to 9. Let's keep going. This is too much fun. 25%, you might recognize
that's the same thing as 1/4, 25% is 25 over 100. If you divide the numerator
and the denominator by 25, you're going to get 1 over 4. So this is equivalent to
saying what's 1/4 of 28. Well, 28 divided by 4 is, of course, 7. Let's keep going. 50% of 128. You might recognize 50%
is the same thing as 1/2. It's 50 over 100, which is equal to 1/2. And so we're really just
saying what's half of 128, or what's 128 divided by 2. And that of course would be 64. And then last but not least, 400% of 8. Well, 400%, that's the
same thing as 400 over 100, or it's equal to 4. So that's really saying what's 4 times 8. So 4 times 8 is, of course, equal to 32. And we are done.