Estimating large sums and differences word problems | 4th grade math (TX TEKS) | Khan Academy

- [Instructor] We're told a
library received 13,246 books

from a book company.

Later that month, they received
another 3,899 books from a

book donation program.

About how many books did the
library receive in total?

So pause this video, have a go at this

and recognize it's saying about,

so you just have to estimate.

You don't have to get the exact number.

Alright, now let's do this together.

So this first video,
sorry, this first number,

I should say, 13,000.

We could round it if we wanna estimate

to the nearest 10,000.

But the second number
is just in the thousand.

So actually I wanna round both of these

to the nearest thousand.

So this first number, I'm
gonna say this is all going

to be roughly equal to,
little squiggly equal signs.

So if I round 13,246 to
the nearest thousand,

it's closer to 13,000
than it is to 14,000.

So it's gonna be 13,000 plus,

if I round the second number
to the nearest thousand,

it's closer to 4,000 than it is to 3,000.

So I'll say plus 4,000, and
we can do this in our head.

13 plus four is 17.

So we're talking about 17,000,

which is this choice right over here.

And once again, there are multiple ways

that you could have approached this

and you just have to
get whichever is closest

to your answer.

Let's do another example.

So here we're told a
farmer picked 35,250 apples

this season.

She sold 4,339 apples at the market.

About how many apples
does the farmer have left?

So pause the video again and
see if you can tackle this.

All right, so once again, it's
about, so we can estimate,

which is always fun to do

and not have to do a bunch of hairy math.

And this number right
here is in the thousands.

This number over here is
in the tens of thousands,

but I like to round to the similar place.

So let's round both to
the nearest thousand.

So this is going to be roughly equal

to 35,250 is closer

to 35,000 than it is to 36,000

and 4,339 is closer to
4,000 than it is to 5,000.

And I want to subtract,

'cause she's selling these,
these roughly 4,000 apples.

And so if 35,000 minus
4,000, that is going

to be 31,000 right over here.

And an interesting thing
to just be aware of is if,

even if you look at these
other choices, if you start

with roughly 35,000 apples,

if you're selling some, there's no way

that your apples should increase.

So if you do some type of estimation,

and there are ways, if you
rounded in a certain way,

if you rounded to the nearest 10,000,

that you actually could end up increasing.

But if you're subtracting,
you should never estimate

with a number that is larger

after the subtraction than the number

that you started off with.

So that one doesn't make sense.

And even if you look at
choice B right over here,

while the difference between 35,250

and 34,400 is less than a thousand,

we're selling way more apples than that.