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

- [Instructor] All right,

we're asked to estimate 87,149

plus 12,227.

And this little curly looking equals sign,

that means approximately equal to,

or what is this roughly equal to?

So pause this video and have a go at this

before we do this together.

Alright, so I always like
the problems with estimation,

'cause it means that I can
kind of do things in my head,

and there's not just one right answer.

But when you have a
multiple choice like this,

you're just trying to find the answer

that's closest to your estimation.

So we have two numbers here,

and they're both, they're
both go all the way

to the 10 thousands place.

So I would feel good, we
could either round this

to the nearest 10,000

or we could round this
to the nearest thousand

if we want to get a little
bit, a little bit more precise.

I'm gonna round to the nearest thousand.

So if we round this first
number to the nearest thousand,

what's it going to be?

Well, it's closer to 87,000
than it is to 88,000.

So I'll just call this
87,000, roughly speaking.

And then 12,227,

once again, if you run
to the nearest thousand,

it's closer to 12,000 than 13,000.

So it's approximately or roughly

going to be equal to 87,000 plus 12,000.

Well, what's 87 plus 12?

Well, 87 plus 12 is 99,

so it's roughly equal to 99,000,

which is exactly one of these
choices right over here.

And even if you got something
a little bit different,

like let's say that you wanted to round

to the nearest 10,000.

So then you would've had
90,000 here plus 10,000 here,

you would've gotten it
roughly equaling 100,000.

Even then, 100,000

would've been closer to this last choice.

Let's do another example.

So pause the video and
have a go at this one.

Alright, now for this one,

the first number is in
the tens of thousands

or it has a 10 thousands place.

But this other number is much smaller.

We only go to the hundreds place,

so it doesn't make sense

to round to the nearest thousand

or round to the nearest 10,000.

And so let's round just
to the nearest hundred.

So if we did that, this
first number right over here,

if we round to the nearest hundred,

it's closer to 34,600
than it is to 34,700.

So let's call this 34,600, roughly.

And then the second number,

if we round to the nearest hundred,

it's closer to 200 than 100.

So this is roughly going
to be equal to 34,600

plus 200, which would be
34,600 plus 200 is 800,

which is once again, exactly
one of the choices here.

But even if you rounded
a little bit differently,

you should probably still
get a similar result.

For example, even if you did
round to the nearest thousand,

which I wouldn't recommend,

but if you did round to
the nearest thousand,

you would've gotten 35,000 here,

and you would've gotten zero over here,

'cause zero, this is closer
to zero than it is to 1,000.

But then you would've said
this is roughly equal to

or approximately equal to 35,000,

which is still much closer
to choice B right over here

than these other choices.