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Multiplication estimation example

Estimating the product of two, 2-digit numbers. Created by Sal Khan.

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Video transcript

A ticket agent sells 42 tickets to a play. The tickets cost $29 each. Use rounding to estimate the total dollars taken in from the sale of the tickets. Now if we wanted the exact number, we could say 42 times 29, and we could work out the multiplication, but they essentially want us to be able to do it in our head. We want to round the numbers first and then multiply. So if we want to round, and really we just have two places here, so if we're going to round anything, it's going to be to the nearest ten because that's the largest place we have. So if we round 42 to the nearest ten-- we've done this drill many times-- 2 in the ones place is the less than 5, so we're going to round down. The nearest ten is 40. We're going to round down to 40. 29, if we round to the nearest ten, 9 in the ones place is greater than or equal to 5, so we round up. The nearest ten is 30. And another way to think about it. Just say, well, you know, 42, that's pretty close to 40. 29 is pretty close to 30. Those are literally the nearest multiples of ten that I can figure out, so now I can multiply. And here, once again, we can use-- you could call it a trick, but hopefully, you understand why it works. But 30 times 40, instead of you saying, well, this is going to be the same thing as 3 times 4, but we're going to put two zeroes at the end of it. 30 times 40 is the same thing as 3 times 4 with two zeroes, so let's do that. So you have 3 times 4 is 12, which we know, and then we have two zeroes. We got that zero, so let's stick that zero there, and then we got that blue zero there, so let's put that over there. So they're going to have roughly $1,200 taken it from sales of the tickets. That is our estimate.