Multiplying by tens word problem | Math | 3rd grade | Khan Academy

A volunteer group is planting trees at five different parks. They planted 90 trees at each park. How many trees did the group plant in all?

So here's what we know: we know that this group went to five different parks, very kind of them, and planted 90 trees at each of those parks. So if we want to know the total amount, we could do a couple of things. We could say they planted 90 trees at Park 1, plus 90 more at Park 2, plus—you see where this is going—90 more at Park 3, plus 90 more at Park 4, and finally, 90 more at Park 5.

One way to solve this would be to add all of those 90s. We could do it; it'll take a while, but we could get there. Or we could say that the group planted 90 trees five times. Five times they planted 90 trees, so 90 * 5.

Let's solve both of these! Either are great ways to get to our solution, but maybe solving both of them will help us see if we liked one way better, if one felt simpler for us.

So, 90—the number 90—we can think of as 9T. I'll write that up there, and that'll help us add.

9T + 9T is 18T, plus 9 more tens is 27T, plus 9 more tens is 36T, and the last nine tens gets us 45T, or 450. That zero is because we're talking about tens. So, the group planted 450 trees.

Now, let's try the multiplication way and see how that goes. So again, 90 we can think of as 9T times 5. So if you have 9 tens five times, you have 45T. And 45T is again 450; get that zero on the end.

Both ways we see this is a very generous group; they planted 450 trees. Either way, we could add or we could multiply. For me, the multiplication gets me there faster. I like the multiplication way, but either way, we can see 90 trees at five parks is 450 total trees.