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Course: Grade 5 (TX TEKS) > Unit 7
Lesson 1: Multiplying decimals and whole numbers- Estimating with multiplying decimals and whole numbers
- Estimating with multiplying decimals and whole numbers
- Multiplying decimals and whole numbers with visuals
- Multiply decimals and whole numbers visually
- Strategies for multiplying decimals and whole numbers
- Multiply whole numbers by 0.1 and 0.01
- Multiply whole numbers and decimals less than 1
- Strategies for multiplying multi-digit decimals by whole numbers
- Multiply whole numbers and decimals
- Reasoning through multiplying decimal word problems
- Multiply decimals and whole numbers word problems
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Reasoning through multiplying decimal word problems
Practice using estimation and thoughtful reasoning to relate multiplying decimals to multiplying whole numbers. Created by Sal Khan.
Video transcript
- [Instructor] We're told that Juan runs 1.7
kilometers every morning. Juan runs the same amount
every day for six days. How many kilometers did
Juan run in six days? Pause this video and see
if you can figure this out before we do this together. All right, so Juan is going
1.7 kilometers per day and he's doing it for six days. So it feels like we should
just be able to multiply by 6. Now in the future we're gonna
learn many different ways of multiplying decimals, but let's just see if we
can reason through this. So instead of just
thinking about 1.7 times 6, we know how to multiply say
something like 17 times 6. So let's do that first and then let's see if
we can reason our way to what 1.7 times 6 is based
on whatever 17 times 6 is. So 17 times 6, we've done this before, or things like this. 6 times 7 is 42. That is 2 ones and 4 tens. And then 1 times 6,
that's really 10 times 6 right over there. That's 6 plus 4, so
then that gives you 10. So 17 times 6 is 102. So based on that, what do
you think 1.7 times 6 is? Well, there's a couple of
ways to think about it. One way to think about
it is you could estimate. You could say, all right,
a whole 6 would be 6, and then you have seven tenths of a 6. So I don't know, that
could be like 4 or 5, someplace in between there. So you would feel that if you have that, this should be a little bit over 10. And if you put the
decimal right over here, that would be a little bit more than 10. So maybe you could say, hey, I'm gonna put the
decimal right over here. So instead of 102, I have 10.2. This seems to make sense, if
we would've just estimated. 2 times 6 is 12, 1 times 6 is 6, and this is going to be closer to 2 times 6 'cause it's 0.7. So this one feels right. Another way that you could
have thought about this is when you go from 1.7 to 17,
you're multiplying by 10. So this number right over here is going to be 10 times larger than this number, and that actually works out as well. So you could divide by 10 right over here, so it would move the decimal place over. Let's do another example of this. The answer is actually 10.2 here. Let's see. The cost to jump at the trampoline park is 8.25 per hour. Riley jumped for four hours. How much money did Riley spend to jump? Pause this video again and see if you can figure this out. Well, similarly, we're gonna multiply $8
and 25 cents times 4. And as I said, in the future,
we're gonna learn ways to do this very almost automatically. But let's reason through it right now. So let's do this first
without the decimals, 825 times 4. And then we're gonna figure out where we might wanna put the decimal. But before I even do that, let's just think about this is, if it was 8 times 4, that would be 32. If it was 9 times 4 it would be 36. So whatever this is, it should
be someplace in between 32 and 36 and it should
probably be closer to 32 'cause we're only a quarter of the way between 8 and 9. So let's just multiply this and then think about where
we could put the decimal to be between 32 and 36. 4 times 5 is 20, regroup the 2. 2 times 4 is 8, plus 2 is 10. Regroup the 1. 8 times 4 is 32, plus 1 is 33. So we get 3,300 here. So what do we think this is going to be? It's going to have the same digits, but where do you think
that decimal's going to be? Well, we already said that
this should be someplace between 32 and 36. And if we wanna do that, well, you can't put the decimal there. That'd be 3,300. You can't
put it there. That'd be 330. You can't put it here. That'd be 3.3. If you put it right over here, it seems to make a lot of sense. And just like we reasoned
through the last example, it does make a lot of sense, because to go from 8.25 to 825, you're moving the decimal over two spots. That's the same thing as
multiplying by a hundred. So this answer is gonna
be a hundred times larger than whatever this answer is. So another way to think about
it, yet you have to undo that. You're going to divide by a hundred and that would get you to the same place that we got just by reasoning through it.