Proof:
Check #1: Check #2:
13 13
x 7 
 7 /28
21  7
+ 7 
 21
28  21

0
Check #3:
13
13
13
13
13
13
+ 13

28 (3+3+3+3+3+3+3) + (1+1+1+1+1+1+1)
In Check #1, multiply 7 x 3 to get 21. Then multiply 7 x 1 to get 7.
Then add 21 and 7 to get 28.
In Check #2, we check the multiplication problem by dividing one factor into the product.
Divide 7 into 2 but it doesn't go.
So divide 7 into 8 to get 1. Subtract 7 from 28 to get 21.
Now divide 7 into 21 to get 3.
So the answer is 13.
In Check #3, another way to check a multiplication problem is by addition.
To check if 7 x 13 = 28, we add seven 13s together.
