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.