The average weight of A, B and C is 45 kg. If the average weight of A and B is 40 kg and that of B and C is 42 kg, then the weight of B (in kg) is:

Correct option is C

Given:

Average(A,B,C) = 45 

A + B + C = 135
Average(A,B) = 40

A + B = 80
Average(B,C) = 42

B + C = 84

Formula Used:
Average =$$\dfrac{\text{Sum of observations}}{\text{Number of observations}}$$​​

Solution:
(A + B) + (B + C) = 80 + 84 = 164 

A + 2B + C = (A + B + C) + B = 135 + B
Now, 135 + B = 164

B = 29 kg.