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## Class 7 (Foundation)

# Multiplication estimation example

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

## Want to join the conversation?

- what is kilometers?(29 votes)
- Kilo means a thousand, therefore a kilometer is 1000 meters.(6 votes)

- math is to hard but khan makes it easier(43 votes)
- at0:05what is rounding?(5 votes)
- Rounding is a type of estimating. 42 is close to 40 = 4 tens. 29 is close to 30 = 3 tens.

Do 4 tens times 3 tens in your head.

4 * 3 = 12 and 'tens place' times 'tens place' = 'hundreds place'

So our estimate is 12 hundred = 1200(15 votes)

- SO i get it a little bit but do you always multiply if so it would be a lot easier and also why do you round. PLEASE ANSWER I'm stuck(7 votes)
- no not always. it depends on what you need to find out, and as far as rounding goes, it's just a way to estimate (guess) when you don't need an exact anwser.(7 votes)

- Couldn't you just round the answer after you got the exact answer?(5 votes)
- You could but the idea is that with rounding the problem become much easier and you may find you can do it in your head.

Sometime we don't always need an exact answer but a good idea, this is why we use rounding.(9 votes)

- What is the point of estimating if its not even the right answer? even if its close, one little mistake will change the whole answer. Do they do it because it is faster?(3 votes)
- when u are rounding something to the nearest dollar estimating can be useful(3 votes)

- Math makes your brain think.(2 votes)
- How do you do this i need help(2 votes)
- U round the numbers and then u times the #(1 vote)

- Anyone know how to out the problem without solving it?(2 votes)
- math is really hard but i get it on khan(1 vote)

## 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.