When to use multiplication

We are told there are 10 students in the poetry club this week. Each student wrote two poems. What does the expression 10 times 2 represent? They give us some choices. Pause this video and see if you can work that out.

All right, so there are 10 students in the poetry club, and each student wrote two poems. So, I would use 10 times 2 to figure out, hey, for each of those 10 students, if they each wrote two poems, then we have 10 times 2 total poems.

If we look at the choices, that is choice C right over here. If we look at choice A, the number of students is 10. It wouldn't be 10 times 2. And then the number of poems each student wrote—well, that's the second piece of information. They each wrote two poems, but that wouldn't be 10 times 2 either.

Let’s do another one of these a little bit different this time. So now we're asked, let me scroll this over a little bit, which problem can we solve with three times eight. So pause this video and have a go at it before we do it together.

All right, now let's read these and see if you can solve it with three times eight. Ellen has eight pieces of gum. Fair enough, she ate three pieces at school. I hope she didn't swallow the gum; that could be problematic. How many pieces of gum does Ellen have left?

So would you use multiplication for that? The way I would tackle it, she has eight pieces of gum and if she ate three and we want to figure out how much gum does she have left (I'm assuming outside of her body), well then I would subtract three from eight. I wouldn't multiply eight by three, so I would rule this one out. That's not going to be my choice. I'm not going to solve this problem with multiplication, or at least by three times eight.

Brendan has eight t-shirts. Fair enough; that's about how many I have. He goes to the store to buy three more t-shirts. How many t-shirts does he have now? Would I solve that with multiplication? Let’s see.

The way I would solve it is, he has eight shirts and then he's buying three more. I would add three to figure out how many total t-shirts he has. I wouldn't multiply by three, so I’d rule that out.

So let’s see how choice C is looking: There are eight chocolates in each box. Mike has three boxes of chocolates. How many chocolates are there?

So this is interesting. He has three boxes of chocolates. So, let me draw those boxes. One box, two boxes, three boxes of chocolate. And each of them have eight chocolates. One, two, three, four, five, six, seven, eight. One, two, three, four, five, six, seven, eight. One, two, three, four, five, six, seven.

Well, yeah, if I want to figure out how many total chocolates there are, I have three groups of eight. I would solve that by multiplying three times eight. So I like this choice a lot.

Let’s do another example here. We are told Gerald is going to plant a garden this spring. That's a good thing to be doing. He plans to have seven plants in each row. There will be eight rows.

Which expression can Gerald use to find the total number of plants needed for his garden? Pause this video and see if you can figure that out.

All right, I’ll do a little bit of a drawing. I’ll do it in green because we're talking about rows of plants. So seven plants in each row and there will be eight rows. So one, two, three, four, five, six, seven—seven plants in each row.

And then we're going to have eight rows. So let me copy and paste that. So, that’s two rows, that’s three rows, that is four rows—getting there, five rows, six rows, seven rows, and eight rows.

So we don’t have to actually calculate it, but the way you could think about it is you have seven plants in each row and then you have eight rows of plants. If you want to figure out the total amount, you would multiply eight times seven.

And so let’s see—that’s this first choice right over here. You would multiply 8 times 7. Times 7 and 8 plus 7 is definitely not going to give you the total number of plants here. So, yeah, we like using the 8 times 7, and we’re done.