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Finding common percentages

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

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.