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. First add up the seven 3s and then add to that the seven 1s.