It does not matter what order you add the numbers:
6 + 3 = 3 + 6

I'm adding 3 + 4 + 7.
I can add the 3 + 7 to make10.
Then, I can add the 4 to make 14.

I am trying to solve
10 - __ = 3 
I can add 10 and 3 to find the unknown number.  

1+9+5 = 15

5+5+9 = 18

40 = 20+10

56 = 65

9 - 3 = 10 - 4

8 = 10 - 3