"Proof" that
7 x 13 = 28

A way to check a multiplication problem is by addition.  Add seven 13's together :

                 13
                 13
                 13
                 13
                 13
                 13
              + 13
             ------
              (3+3+3+3+3+3+3) = 21 & (1+1+1+1+1+1+1) = 7 ;   21 + 7 = 28

or

  13
x  7
-----
  21            -- because 7 x 3 = 21
+  7            -- when you multiply the 7 times the 1
-----
 28
 

**from: http://www.pen.k12.va.us/Div/Winchester/jhhs/math/humor/mult.html