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.