Adding and subtracting 2-step word problems | Khan Academy

- [Instructor] A school
secretary started the school year

with 450 markers.

At open house, I should
say at the open house.

At the open house parents
donated 163 markers.

By winter break, 278 markers
had been dried out or lost.

How many usable markers
does the school have now?

Pause this video and try
to think through that

before we do this together.

Alright, let's first try to visualize this

with a strip diagram.

So we start the school
year with 450 markers.

So let's say that this is,

and I'm just eyeballing it so
it's not going to be exact,

but just to help visualize.

So let's say that this
is 450 right over here.

So that's the number of
markers that we start with.

And at the open house,
parents donated 163,

so that's going to increase
the number of markers we have.

So lemme do this in another color.

So they donated 163 markers,

so we should increase by 163.

So it looks something like this.

We should increase by another 163.

And then by winter break,

278 markers had been dried out or lost.

So this is now taking away from
our total number of markers.

278 had been dried out or lost.

So we could take those away.

The 278, maybe it looks
something like this.

278.

And so this piece right over here

is what is going to be left over.

So we need to figure out this
thing that I'm shading in,

I guess you could say black.

So first we could add 450
to 163 to get this total.

And then we could subtract 278.

So we're going to do 450 plus 163,

plus 163.

And then we're going to subtract 278.

So first 450 plus 163,
I can expand those out.

That's the same thing.

I'll do it.

Let's see, I think I have space here.

400, at four hundreds plus five tens,

that's 50, plus zero ones.

And then I'm adding 163,

that's 100, six tens and three ones.

And if I add those two
together, maybe in purple,

I'm going to get three ones.

I am getting 50 plus 60,

that's 110 or 11 10s.

Five plus six is 11, but
we're in the tens place.

So this is 11 10s or 110,

and then 400 plus 100 is 500.

So 500 plus 110.

This part over here is 610 plus three,

which is equal to 613.

So this entire thing right over here,

this whole thing is 613.

And now we wanna subtract 278 from that.

Well, we could use a similar technique.

We could expand out, 613 is
six hundreds plus one 10,

plus three ones.

And we're gonna subtract out two hundreds,

seven tens, and eight ones.

So minus 200, subtracting seven tens,

and subtracting eight ones.

Now when we do this,

we can't subtract three
from, eight from three.

So let's take 10 from the tens place.

So this becomes zero.

We put the 10 here, this becomes 13.

13 minus eight is five.

Now we have a problem in the tens place.

We have no tens and we wanna subtract 70.

So let's take a hundred
from the hundred's place.

So if we take a hundred,

this becomes 500 and we put
that a hundred over here,

this becomes a hundred,

and I get a hundred minus 70 is 30,

and then I have 500 minus 200 is 300.

So if 300 plus 30 plus five,

which is three hundred and thirty five,

three hundreds, three tens, and five ones.

So that's how many usable markers

the school now has, 335.