My maths thinking

15 + 6 =21                              15 + 6 =21

  +  / \                                         +  / \     

   3 + 3                                         5 +1

15 + 3=18                                   15 + 5=20                  

18 + 3 =21                                  20 + 1 =21

I split the 6 into 3 and a 3 and I gave                      I split the 6 into a 5 and 1.

3 to the 15 to make 18 .Then I gave the other       Then I gave the 5 to the

3 to the 18 that   made 21.                                           15 to make 20.Then I gave the 1 to the      
20 to make 21.