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## Topic C: Multiplication of up to four digits by single-digit numbers

Current time:0:00Total duration:4:58

# Multiplying with area model: 78 x 65

CCSS Math: 4.NBT.B.5

## Video transcript

I'm going to multiply 78
times 65 in a little less than standard way, but
hopefully it'll make some sense, and you'll realize that
there's multiple ways that you can multiply. And this is actually the
way that I multiply numbers in my head. So 78 times 65. And then we're
actually going to think about what the different
parts of this process represent on this area model. So 78 times 65. So I'm going to start
just the way we normally start when we multiply. I'm going to start with
this 5 in this ones place, and I'm going to
say 5 times 8 is 40. And instead of just
writing a 0 and carrying a 4 right over here, I'm just
going to write the number 40. So this was the 5 times the 8. Now, I'm going to multiply
the 5 times the 7. And we have to be a
little bit careful here because 5 times-- this isn't
just any 7, this is a 70. So what is 5 times 70? Well, 5 times 7 would be
35, so five times 70 is 350. So I'll write that down, 350. So just as a review, 5 times
8 is 40, 5 times 7 is 350. If you add these two together,
this is going to be 5 times 78. Now let's go over to the 6. So let's multiply
the 6 times the 8. Now we have to be careful again. This 6 is not just a regular
6, it's in the tens place. This is a 60. 60 times 8. Well, 6 times 8 is 48, so 60
times 8 is going to be 480. So it's going to be 480. And then 6 times 7. Well, that would be 42,
but we have to be careful. This is 60 times
70, so we're going to have two zeroes at the end. This is 4,200, not just 42. So 6 times 7 is 4,200. And now we can add
everything together. And this is a very
similar process to what we do when we do
the traditional method of multiplying. I just made it a little
bit more explicit what parts are from
multiplying which digits. But we can add
everything together. In the ones place, we have a 0. In the tens place, we
have 4 plus 5 is 9. 9 plus 8 is 17. And now we can carry a 1. 1 plus 3 is 4. 4 plus 4 is 8. 8 plus 2 is 10. Carry a 1, regroup
of 1 even, and then you have a 5 right over there. So you get 5,070. Now, I want to
think about-- I want to visualize what was going
on here using this area model. So once again, we had 78. So I'm going to make this
vertical length represent 78. So this distance right over
here represents the 70. That's the 70, and then we'll
make this distance right over here represent the 8. Let me make that a
little bit cleaner so you see what
I'm talking about. So this distance right
over here represents the 8 and then we're going to
multiply that times 65. So this distance
right over here-- it's not drawn
perfectly to scale, but it gives the
idea-- this is 60. And then this distance
right over here is the 5. So this whole distance is 65. This entire distance is 78. So when you multiply-- if
you had a rectangle that's 65 units wide and
you multiplied it, and it had a height of
78 units, then its area is going to be 78 times 65. It's area is going to be 5,070. Now, each of these
parts we can map to one part of this area
model right over here. When we multiplied
the 5 times the 8 and got the 40, that was
this section right over here, 5 times 8 is equal to 40. When we multiplied 5
times 70, and got 350, that's this right over here. 5 times 70, and we got 350. When we multiplied 60-- the 6
in the tens place-- 60 times 8 and got 480-- that's
this right over here-- this is 60 times
8 is equal to 480. And then finally when we
multiplied 60 times 70 and got 4,200-- that's
this area right over here-- this area is 60 wide, 70 tall. So this is 60 times 70,
which is equal to 4,200. And then when we added
everything up to get 5,070, we were essentially
just adding up the areas of each
of these tiles. This big one is 4,200, that
makes up most of the area, and we get 480 from this
magenta one, then 350 from this greenish-yellow
one, and then 40 from this greenish-blue
one to get 5,070. So the area of this entire
thing is 5,070 square units.