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### Course: Operations and Algebraic Thinking 189-200>Unit 4

Lesson 4: Multiplying by tens

# Multiplying by tens word problem

Lindsay multiplies 90 times 5 to solve a word problem. Created by Lindsay Spears.

## Want to join the conversation?

• so would this question be right? 50x4=200?
• Yes, you're right.
Here are my steps, broken down:
50 ×4
5×10 ×4
Here you can switch the order of the numbers due to the commutative property:
5×4×10
We know 5×4=20, so 5×4 ×10 = 200.
• isnt 90 times 5 the same as adding 90 5 times?
• Multiplication is just that.
• so am I right 500x4=2,000
• Yes, that's right!
• You know multiple right 😨
• Guys does 70 times 5 equal 7 times 5 (which is 35) plus a 0 at the end?
• Yup, that's exactly right.
Reasoning: `7``0 * 5 = (7``*5)``*``10 = 350`
By the way, did you mean append a `0` to the end, like `35 -> 35``0`?
"Plus" sounds like `+ 0` which doesn't do anything.
• Sal is better make Sal in more videos
• i want to see what he looks like RIGHT NOW
(1 vote)
• this isnt a question but did anyone do it this way: so you think 90 x 5 so 90 is 9 x 10 so its 9 x 10 x 5 so 9 x (10 x 5) so its 450
(1 vote)
• did you know about ! in math
(1 vote)
• That symbol of '*!*' or the exclamation point in math can also be known as factorial. Which basically means multiply all the numbers until that select number, which means that those numbers tend to be quite big. Let us try 5!:

``1 x 2 x 3 x 4 x 5 = 5! = 120``

So, now, 5! is = 120.

Hope this helps!
(1 vote)
• bro this is teaching me a lot of things
(1 vote)

## Video transcript

- [Voiceover] 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? 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. If we want to know the total amount, we could do a couple things. We could say they planted 90 trees at park one, plus 90 more at park two, plus, you see where this is going, 90 more at park three, plus 90 more at park four, and finally 90 more at park five. One way to solve this would be to add all 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 times five. Let's solve both of these. Either are great ways to get to our solution, but maybe solving both of 'em will help us see if we liked one way better or if one felt simpler for us. So, 90, the number 90 we can think of as nine 10s, I'll write that up there, and that'll help us add nine 10s plus nine 10s is 18 10s, plus nine more 10s is 27 10s, plus nine more 10s is 36 10s, and the last nine 10s gets us 45 10s, or 450. That zero is because we're talking about 10s, 45 10s, or 450. The group planted 450 trees. Let's try the multiplication way and see how that goes. Again, 90 10 90, we could think of as nine 10s times five, so if you have nine 10s five times, you have 45 10s, and 45 10s 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.