The sum will be prime if its 2, 3, 5, 7, 9, 11, or 13
0+2=2
1+2=3
5+2=7
9+2=11
0+3=3
1+4=5
9+4=13
0+7=7
That's 8 combinations, and there 4*4=16 total.
8/16 = 1/2.
