Solving multi-step problems (multiplication): firewood | Khan Academy

- [Instructor] We're told a camping group

gathers three bundles of firewood

with 17 logs in each bundle.

Later they use nine logs
to start a campfire.

How many logs are left,

I guess after they use those nine logs?

So pause this video and have a go at this

before we do this together.

All right, now let's
work on this together.

So the first thing I
think about is, all right,

the camping group started
with three bundles

and each of those bundles had 17 logs.

So the amount that they started off

with is going to be three...

Let me do the same colors.

So they would've started
off with three times 17.

So this right over here is
what they're starting out with.

And so we'd wanna figure that out first.

So this is starting amount, starting.

And then you'd wanna
subtract out what they used.

And we see that they used nine logs.

So then we wanna subtract out those nine.

So let's do that computation.

So first let's do 17 times three.

I'll do that over here on the side.

So 17 times three,

times three.

We'll go to a neutral color here.

So the once placed
seven times three is 21.

We'll regroup those two 10s
of the 21 into the 10s place.

And then one times three is three.

It's actually 30,

'cause we're talking about three 10s.

And then we have these two
more 10s, and so we get 51.

So the starting amount is 51.

And then from that we
are going to subtract,

going to subtract nine.

Now, some of you might be
able to do this in your head.

My head would say, all
right, if I did 51 minus 10,

it would be 41, but I'm
subtracting one less than this,

so this is going to be 42.

But if you wanted to do it
using the traditional way,

you could say 51 minus nine.

Minus nine like this.

You start at the ones place

and you say, well, I can't
subtract nine from one,

but I can regroup.

I can take 10 from the 10s
place, so I have four 10s left.

And when I write those 10,

that one 10 in the ones
place, it becomes 10.

So it's 10 plus this one
right over here, so it's 11.

11 minus nine is two,

and then four tens minus
no tens is four tens, 42.

So how many logs left?

42.