Multiplying 2 digit by 1 digit numbers within 100
About this video
The video finds 3 x 29 with an area model. It splits a 3 by 29 rectangle into a 3 by 20 part and a 3 by 9 part, finds 60 and 27, and adds them to get 87.
You'll be able to
- draw an area model for a 2-digit times 1-digit multiplication.
- split a number into tens and ones, like 29 = 20 + 9.
- add the partial areas to find a product, like 60 + 27 = 87.
Key words
- area model
- A rectangle drawing where the side lengths are the numbers you multiply and the area is the answer.
- area
- The amount of space inside a shape, measured in square units.
- square units
- Little squares used to measure area.
Transcript
- [Instructor] What I wanna do in this video is try to figure out what 3 times 29 is. And there's a lot of ways of approaching this, but the way that we're going to think about it is using an area model. What do I mean by that? Well, we can imagine a rectangle that is 3 units long in the vertical direction, or in that direction, and then 29 units in this direction, and this isn't drawn completely to scale.
And if you were to multiply what 3 times 29 is, you would get this area right over here. So how can we use this area model? Well, one way to do it is to think about 29 as being the same thing as 20 + 9. So, for example, let's say that this part right over here is 20 units long, in the blue.
So I'm gonna split up this rectangle. So that part right over here is 20 units long, and this part right over here is 9 units long. So this is 9, and, to be clear, this part right over here is 20. And it's still 29 units long, or wide, because 20 + 9 is 29. Now the reason why this is useful is I can now think about these different areas.
I can now think about this area right over here. It's 3 units high and 20 units wide. So what's this area? Well, this area is going to be 3 times 20, which we can do in our head, 3 times 2 is 6, so 3 times 20 is 60 of these square units. And then we could also think about this area right over here.
This area is going to be 3 times 9, which you might already know is 27. So if you wanna figure out the entire area here, which would be 3 times 29, you would just add the 60 plus the 27, so I could write it here maybe, this is 60 + 27, which is going to be equal to, 60 + 20 is 80 plus another 7, is going to be 87.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.