Estimating large differences | 4th grade math (TX TEKS) | Khan Academy

- [Instructor] All right,
we're asked to estimate

29,224 minus 18,793.

This squiggly looking equal sign,

that means roughly equal to
or approximately equal to.

So pause this video, have a go at this

before we do this together.

All right, so the way I think about it,

both of these are in
the tens of thousands.

We could round to the nearest 10,000.

If we wanna get a little bit more precise,

we could round to the nearest thousand.

So let's go with the nearest
thousand, see what happens.

So let's see, 29,224 is closer to 29,000

than it is to 30,000, so let's
say this is approximately

or roughly going to be
equal to 29,000 minus,

now, 18,793, that's closer to 19,000

than it is to 18,000,

so let's say that that's roughly 19,000.

And this we can do in our
head, 29 minus 19 is 10.

This is 29,000 minus 19,000,

so this would be 10,000 right over here,

which is exactly one of these choices.

Now, you might not get exactly this,

but you should just get when you estimate,

'cause there's many
different ways to estimate,

whichever is closest.

For example, you could have
rounded to the nearest 10,000.

In that case, you would've said

that this is roughly equal
to 30,000 minus 20,000,

which would've actually
also have given you 10,000.

So that's just a case
where even if I do it

two different ways,

I still would've gotten that same answer.

Now let's do another example here.

So here we're told to
estimate 76,371 minus 2,842.

Pause the video and have a go at that.

All right, well, these,

and 2,842, I'm not even
in the tens of thousands,

so maybe I just wanna
definitely round here

to the nearest thousand,

so this is closer to
3,000 than it is to 2,000.

So let's say that this is,
well, lemme do the 76,000 first.

If I round to the nearest thousand,

it's closer to 76,000 than 77,000,

so this is roughly equal to 76,000 minus,

and the second number is
closer to 3,000 than 2,000,

and so if I subtract, we
can do that in our heads,

that is going to be 73,000,

which is exactly one
of these choices here.