6 + (a)   = 13

(a)   + 5 = 15

(a)   + 8 = 13

9 + (a)   = 12

9 + (a)   = 15

8 + (a)   = 11

(a)   + 4 = 13

(a)   + 6 = 13

(a)   + 8 = 12

8 + (a)   = 15

9 + (a)   = 14

(a)   + 4 = 12

(a)   + 8 = 15

7 + (a)   = 13

6 + (a)   = 16

5 + (a)   = 11

(a)   + 4 = 11

(a)   + 9 = 11

6 + (a)   = 14