Correct option is B
Given:
24 B 6 B 12 = 42
31 B 7 B 13=51
64 B 9 B 4 =?
Let: B = +
After replace the sing we get following equations-
24 + 6 + 12 = 42
42 = 42
And
31 + 7 + 13=51
51 =51
Similarly,
64 + 9 + 4 =?
? = 77
Correct answer is (b) 77.