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# Multiplication estimation example

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

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.