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### Course: Class 4 > Unit 2

Lesson 3: Multiply by 1-digit numbers# Multiplying 4-digit by 1-digit (regrouping)

Learn to multiply a 4-digit number by a 1-digit number. In this video, we will multiply 8085 times 9. Created by Sal Khan.

## Want to join the conversation?

- can u do the same thing or concept for billion(5 votes)
- Yes you can. The possibility of multiplying different numbers is infinite.(8 votes)

- why are there negitive numbers?(5 votes)
- You can imagine that negative numbers mean something owed. It's not that you can hold negative something, but rather you need to take that number from something else. For example I have 25 apples and you want 5. So 25-5 = 25 + (-5) represents you taking the 5 from me.(7 votes)

- I'm surprised no one uses this for chat(7 votes)
- you don't need to use this for chat because u probably don't even know anybody here and it's no fun(1 vote)

- if you change the places will you need to do more work then you orignaly did (im bad at spelling sorry)(5 votes)
- Yep! It's ok that you aren't good at spelling as long as others can understand you, and I can.(4 votes)

- so 5x2 is like 2+2+2+2+2 so 5x2=10 so 2+2+2+2+2 ok i can do that?(5 votes)
- Yes. That is a common strategy for multiplication.(1 vote)

- i need to learn my fact in my head(4 votes)
- That's what i do but it gets diffucult in your head and it messes up the problem(2 votes)

- Do you sometimes need to divide when you're regrouping,or is it just plain multiplying?(3 votes)
- the multi digit multiplication is so easy and now i under stand it better than before.(2 votes)
- what is 90,99999999x5000,999999=

what.(2 votes)- your answer is 455090.999859(2 votes)

- whats 1,000,000 digit by 1,000,000 digit?(2 votes)
- The answer to this calculation 🧮 is 1,000,000 squared ( 1million squared )

Hopefully, this helps!(2 votes)

## Video transcript

Let's multiply 9 times 8,085. That should be a pretty fun
little calculation to do. So like always, let's
just rewrite this. So I'm going to write the 8,085. I'm going to write
the 9 right below it and write our little
multiplication symbol. And now, we're ready to compute. So first we can
tackle 9 times 5. Well, we know that
9 times 5 is 45. We can write the 5
in the ones place and carry the 5
to the tens place. So 9 times 5 is 45. Now we're ready to
move on to 9 times 8. And we're going to
calculate 9 times 8 and then add the 4
that we just carried. So 9 times 8 is 72,
plus the 4 is 76. So we'll write the 6 right here
the tens place and carry the 7. Now we are ready to
calculate, and I'm looking for a suitable color. 9 times 0 100's plus-- and this
is a 7 in the hundreds place, so that's actually 700. Or if we're just kind of
going with the computation, 9 times 0 plus 7. Well, 9 times 0
is 0, plus 7 is 7. And then, finally, we
have-- and once again, I'm looking for a suitable
color-- 9 times 8. This is the last thing
we have to compute. We already know that
9 times 8 is 72. And we just write the 72 right
down here, and we're done. 8,085 times 9 is 72,765. Let's do one more
example just to make sure that this is really clear
in your brain, at least the process for doing this. And I also want you to
think about why this works. So let's try 7 times 5,396. And I encourage you to pause it
and try it on your own as well. I'm going to rewrite
it-- 5,396 times 7. First, we'll think
about what 7 times 6 is. We know that's 42. We'll put the 2
in the ones place. 4 we will carry. Then we need to concern
ourselves with 7 times 9. But then, we have to calculate
that and then add the 4. 7 times 9 is 63, plus 4 is 67. So we put the 7 down
here and carry the 6. Then we have to worry
about 7 times 3 plus this 6 that we had just
finished carrying. 7 times 3 is 21, plus 6 is 27. So we'll write the 7 here in the
hundreds place and carry the 2. And then, finally, we have
7 times 5, which is 35. But we have to add the 2. 35 plus 2 is 37. So 5,396 times 7 is 37,772.