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## 5th grade foundations (Eureka Math/EngageNY)

### Course: 5th grade foundations (Eureka Math/EngageNY) > Unit 4

Lesson 4: Topic D: Foundations# More ways to multiply

Sal uses arrays and repeated addition to multiply. Created by Sal Khan.

## Want to join the conversation?

- hi sal, ive learnt soo much from you, I'm not that good with math but after joining khan academy I know how to multiply 10s, 100s, ect. add, subtract, and decimals, but I am really struggling to memorise the multiplication table, any tips?(78 votes)
- well write it down a few times that is how i learned it.(32 votes)

- What if you have 0 group(19 votes)
- Its just 0 because that is kind of like a 0(14 votes)

- Vote me and I'll vote you(4 votes)
- what if you don't know how to skip count by 8 than what do you do?(2 votes)
- Remember that 8 is the same as 2 groups of 4. (8 = 2 x 4) If you can't count by groups of 8, try counting by groups of 4. If that is still too hard, try counting by groups of 2 and work your way up to counting by larger numbers.(0 votes)

- Can you do standerd algridam(1 vote)
- How do you play fortnight(1 vote)
- Example of decamils 4.05676(1 vote)
- Change you grammar Sal.(1 vote)
- the only way i could see the picture was by looking in a window of a mirror in a window and looking out the windows of a mirror and then looking up and looking up and seeing what was happening and then looking up and looking up to the sky in a different window of a mirror in the same room and then seeing the light from behind it is amazing to look back on the light and look at the sky and see the light coming out from the sky and the sky and the stars from above and above and the sky from above it all the light way(1 vote)

## Video transcript

If we have 2 groups and
in each group I have 4, so that's one group
of 4, and then here is my second
group of 4, we already know that we could
write this as 2 times 4, which is the same
thing as 4 plus 4. Notice I have two 4's here. I have 4 plus another 4. Well, if I have 4 plus 4,
or if I have 2 groups of 4, either way, I'm going to
have a total of 8 things. And you see that
right over here. We have 1, 2, 3, 4,
5, 6, 7, 8 things. What I want you to do
is pause the video now and try to group these
same 8 things, but to group it in other ways so
that we can represent 8 as the product of whole numbers. Here I've represented 8
as the product of 2 and 4. 2 times 4 is 8. See if you can represent
8 as the product of other whole numbers,
or as whole numbers in different ways, grouping
it in different ways. So I assume you've
paused the video. So let's try it out ourselves. So one thing we
could do, we could view this instead
of as 2 groups of 4, we can view 8 as 4 groups of 2. So that's 1 group of 2, 2
groups of 2, 3 groups of 2, 4 groups of 2. So we could write that
4 times 2 is equal to 8. And we could view this as
the same thing as, literally, 4 2's. We have one 1, 2, 3, 4 2's. Each of these have 2 in them. So we're going to
say 1, 2, 3, 4 2's. 2 plus 2, plus 2,
plus 2 is equal to 8. These are both equivalent. 4 times 2, literally
4 groups of 2. That's the same thing as taking
4 2's and adding them together. Notice, we have 2
2's right over here. We added them together, 1, 2. Here, we have 4 2's
and we're adding them together, 1, 2, 3, 4. We take our 4 2's and
we add them together. How else could we represent 8? Well, we literally could
view it as 8 groups of 1. So let's do that. So 8 groups of 1
would look like this. That's 1 group of 1,
2, 3, 4, 5, 6, 7, 8. So we could write this
down as 8 times 1. 8 times 1 is, once
again, equal to 8. And if we wanted to write this
down as repeated addition, well, this is literally 8 1's. So 1 plus 1, plus 1,
plus 1, plus 1, plus 1. Let's see. That's 1, 2, 3, 4, 5, 6, 7, 8. 1, 2, 3, 4, 5, 6, 7, 8. 1 plus 1, plus 1, plus 1,
plus 1, plus 1, plus 1, plus 1 is equal to 8. Now, you might be
a little stumped. Well, what's another
way of getting to 8? Well, you could literally
view it as 1 group of 8. So let me view it that way. So this is just 1 entire
group of 8, the whole thing. The whole thing is a group of 8. So let me scroll over to
the right a little bit. We could write this
down as 1 times 8. And 1 times 8 is equal to 8. And how would we view that? Well, we only have one 8 now. We don't have to add that
one 8 to anything else. So if we wanted to do it the
way we've done the last few, we could literally write it
down as we just have one 8. Well, one 8 is clearly
going to just be equal to 8. So now let me ask
you another question. So far we've been focused
on each of these groups, but what if we actually
view this as 4 groups of 8. Then how many things are
we actually going to have? So let me make this very clear. So we have 1 group of eight 8,
2 groups of 8, 3 groups of 8, and 4 groups of 8. So we would view this
as 4 times 8, or which is going to be the same thing
as 8 plus 8, plus 8, plus 8. 4 8's. What is this going
to be equal to? A And I encourage you
to pause the video and figure it out right now. Well, there's a couple
of ways that you could have thought about this. You could have literally
just counted these. Or you could say, well, let's
see, you can skip count by 8. 8, 16, 24, 32. Or you could have said, 8
plus 8 is 16, plus 8 is 24, plus 8 is 32. Or you could have literally
just counted the triangles here.