The difference between the total number of boys in A and B = The difference between the total number of boys in A and C (The total number of boys in C > A)
C - A = B - A
C = B = 48
The total number of girls in C = 84 - 48 = 36
The total number of girls in B = 36 - 6 = 30
The total number of girls in A = 30 - 6 = 24
The total number of boys in A = 64 - 24 = 40
The total number of girls in C = 36
The total number of girls in B = 30
The total number of girls in A = 24
Hence, the required average = (36 + 30 + 24)/3 = 30