# Multiplying: 2 digits numbers (using distributive property) | 4th grade | Khan Academy

In this video, I’m going

to multiply 87 times 63. But I’m not going to do it

just by using some process, just showing you some steps. Instead, we’re just going to

use the distributive property to actually try to

calculate this thing. So first, what I’m going

to do– let me rewrite 87. So this is the same thing as 87. But instead of

writing 63 like that, I’m going to write

63 as 60 plus 3. Now, what is this

going to be equal to? Well, 87 times 60

plus 3, that’s going to be the same thing

as– and let me actually copy and paste this. So this is going to be the

same thing as 87 times 60 plus 87 times 3. You could say that we’ve

just distributed the 87. We’re multiplying

87 times 60 plus 3. That’s 87 times 60

plus 87 times 3. I could put parentheses here to

make it a little bit clearer. Well, fair enough. But then how do you

calculate what this is? Well, now we can

rewrite 87 as 80 plus 7. So let’s rewrite that. So this is the same thing. Actually, let me

write it this way. I can swap them around. So this is the same

thing as 60 times 87. But I’ll write that

as 60 times 80 plus 7. We could do it like this. 80 plus 7 plus 3 times

80 plus 7, or 3 times 87. Let me just copy

and paste that, so I don’t have to keep

switching colors. Plus 3 times 80 plus 7. So copy and then

let me paste it. And then you have

it just like that. So all I did, just to

be clear– all of what you see right over here,

87 times 60, well, that’s the same thing as

60 times 87, which is the same thing as

60 times 80 plus 7. All that you see

here, 87 times 3, that’s the same thing

as 3 times 87, which is the same thing as

3 times 80 plus 7. That’s just that over here. But look, we can

distribute again. We can distribute the

60 times 80 plus 7. So this is going to be 60– I’m

going to do that same color. Color changing is hard. This is 60 times 80 plus

60 times 7 plus 3 times 80 plus 3 times 7

right over here. So notice what we

really did is we thought about what each

of these digits represent. 8 represents 80. 7 represents 7. 6 represents 60, because

it’s in the tens place. The 8 was in the

tens place, as well. This 3 is in the ones

place, so it’s just 3. And we just multiplied

them all together. We multiplied the

80 times the 60. We multiplied the

80 times the 3. We multiplied the 7 times

the 60 right over here. We multiplied the 7 times the 3. And then we add them

all up together. And this will actually

give us our product. So for example, this right

over here, 6 times 8 is 48. But this isn’t six 8’s. This is 60 80’s. So this is going to be 4,800. We’ve got two 0’s

right over here, so 48 followed by the two 0’s. This right over here,

60 times 7, is 420. 6 times 7 is 42. But it’s going to be 10 times

as much, because this is a 60. And then 3 times 80–

well, same logic. 3 times 8 is 24. So this is going to be 240. And then, finally,

3 times 7 is 21. And then to get the product,

we can add these two together. And you might be

saying, hey, Sal, I know faster ways

of doing this. But the whole reason

I’m doing this is to show you that that fast

way you knew how to do it, it’s not some magical formula

or some magical process you’re doing. It just comes out of really

the distributive property and, hopefully, a little

bit of common sense. So what is this

going to be equal to? Well, we could add them all up. 4,800 plus 420 plus 240 plus 21. Well, you’re going

to get a 1 here. Let’s see. 20 plus 40 plus 20 is 80. Let’s see. 800 plus 400 is 1,200

plus 200 more is 1,400. And so you get 5,481. It’s equal to 5,481. And you might say, gee,

this was a bit of a pain to have to do the distributive

property over and over again. Is there a simpler way

to maybe visualize this? And there is. You could actually

write this as a grid. So we could say we’re

multiplying 87 times 63. We could write it like this. We could say it’s 80

plus 7 times 60 plus 3. And then you can set

up a grid like this. So let me set up

a little box here. It’s 2 digits by 2 digits. So it’s going to be a 2 by 2

grid, 2 rows and 2 columns. And then you just

have to calculate. Well, what’s 60 times 80? Well, we already

calculated that. That’s 4,800. What is 60 times 7? Well, that’s going to be 420. What is 3 times 80? We already calculated that. That is 240. And I want to do that

same color– 240. And finally, what is 3 times 7? 21. You add them all together. You get 5,481. And I encourage you to now just

do this same multiplication problem, the same

87 times 63, the way that you might have

traditionally learned it. And look at the different steps

and why they are making sense and why, at the end

of the day, you really are doing the same thing that

we just did in this video. You’re just doing it

in a different way. And the whole point of

this whole exercise, this whole video, is so

you’re not just blindly doing some type of steps to

find the product of two numbers. But you can actually

understand why those steps work and how those numbers

relate to each other.

1. 😀

Did Sal literally make this video a few minutes ago?

3rd comment

Is this the first step in understanding how to multiply two-digit numbers in your head? I'm a little slow, but I could do this in my head just knowing the multiplication tables and how to re-group the digits, then multiply, then add the results.

Hey I hate all the homework I get solve that

Sal I wish you could give me a high five right now

Nice! It's always fun to look at things a little different.

Hi Sal, can you please make a video about directional derivatives.

It would make me very glad 🙂

12 whats up please answer

Colorful!!! 🙂

Thanks for Khan Academy!!!

I always use it!!!

So helpful!!!:)

to did she get the 7

C.O.O.L

CONSTAPATED

OVERPOOPED

OLD

LADY

= COOL

thank you khan academy

Makes me more understanding

you are smart.

Very Good!!!

thanks! this really helped me!

4th grade? my grade 8 friends struggle on this type of stuff

37×28 can't work out can someone help please?

What program did you use to write the letters out? By the way this helped me a lot

63×7+70×63

I’m trying to do it like 7x+7y

THANKS KHAN

ACADEMY!

you are good at math

That helped me a lot thanks

Thnx a lot its useful to me

So helpful

nooooooooooooo

Can someone help me 7 x 256 am sorry am dum

thanks

Too long process but good

LOVE THIS STUFF!

im still confused…

and im in 5th…

I wasnt taught this in class today

I don’t understand new math this makes no sense.

Am I he only kid watching this cuz I didn’t understand my homework

Me tooooo

Anybody who dislikes khan academy doesn't like learning.

You should draw the sides of the rectangle grid to more closely match the numbers. For instance, since 80 plus 7 is along the top of the grid, it makes more sense to have the line representing 80 to be longer than the line representing 7. This is an "area model" using length times width. 🙂 Then do the same thing with the numbers going along the left side. That is the way that helps students relate the multiplication to geometry as "area."

Way to confusing

🙁

1/10

Khan Acedemy literally has saved my life so many times.

Sir, please make videos on Vedic maths (especially DIVISION)!!

Thank you so much this helped me with my homework so much

HOW IS THIS 4TH GRADE? I’m in grade 6 and I don’t get it and we’ve never learned it before.

What program is he using

I swear he always saves me from a test.

Thx!

what program did they use to draw all of that because i like it

Thanks very much bro

Thank you khan academy for this video!!

♡º♡º♡

I don't understand my homework too!

yay me to

Your helping me with the d property for homework

thanks bro your a real lifesaver