Here we have,

As it’s mentioned that there is no carry-over addition, then

1 + C = 6

⇒ C = 5

A + C = 9

⇒ A + 5 = 9

⇒ A = 4

B + A = 7

⇒ B = 7-A = 7-4 = 3

Therefore, the value of A + B + C = 4 + 3 + 5 = 12