Video: Estimating a Part given the Whole and Its Percentage

Estimate the following: 21% of 50.

01:40

Video Transcript

Estimate the following: 21 percent of 50.

A good example of needing to use this in real life is going out to eat. When calculating a tip, we are supposed to take the bill amount and multiply it by a percentage. And the tip amount is up to us, usually between 15 and 20 percent.

Now notice it says to estimate. So instead of using 21 percent, we can estimate using a number that’s a little easier to work with, 20 percent. Now the word “of” in mathematics means to multiply. So 20 percent times 50, 20 percent we can rewrite as 20 out of 100 and 50 we can rewrite as 50 over one.

Now since we’re estimating, maybe we didn’t have a calculator. So instead of multiplying straight across and have a very large numbers, we can simplify here, actually two different options of simplifying. We can simplify the 20 into the 100. Or we can simplify the 50 into the 100 because we can take a number on the top and simplify it with a number on the bottom.

Let’s say we did 20. 20 goes into itself once and goes into 100 five times. 50 times one on the numerators is 50. And five times one on the denominators is five. And 50 divided by five is 10. Now let’s say we simplify the 50 in the 100 instead. 50 goes into itself once and 100 twice. So on the numerators, 20 times one is 20. And on the denominators, two times one is two. And 20 divided by two is again 10. Therefore, our estimate is 10.

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