Answer
C + 5 = 11
∴ C = 11 - 5 = 6
And 8 + B + 1 = 15
∴ B = 15 - 9 = 6
And A + 5 + 1 = 13
∴ A = 13 - 6 = 7
And 6 + D + 1 = 9
∴ D = 9 - 7 = 2
Hence A = 7, B = 6, C = 6 and D = 2