Solving multi-step problems (multiplication): firewood
About this video
A camping group gathers 3 bundles of firewood with 17 logs each and then uses 9 logs. The video multiplies 3 x 17 = 51 to find the starting amount, then subtracts 9 to get 42 logs left, showing both a mental strategy and the regrouping method.
You'll be able to
- decide to multiply first and then subtract in a two-step word problem.
- multiply a two-digit number by a one-digit number with regrouping.
- subtract with regrouping, or use a mental strategy like 51 - 10 + 1.
Key words
- bundle
- A group of things tied together, like logs of firewood.
- regroup
- To trade 10 ones for 1 ten, or 1 ten for 10 ones, when adding, subtracting or multiplying.
- starting amount
- How many you have at the beginning, before anything is used up.
Transcript
- [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.
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