The average weight of A, B and C is 45 kg. If the average weight of A and B be 40 kg and that of B and C be 43 kg, then the weight of B is___________?

A. 17 kg
B. 20 kg
C. 26 kg
D. 31 kg

Explanation:

Let A, B, C represent their respective weights.

Then, we have:
A + B + C = (45 * 3) = 135 — (i)
A + B = (40 * 2) = 80 — (ii)
B + C = (43 * 2) = 86 — (iii)
Adding (ii) and (iii), we get: A + 2B + C = 166 — (iv)
Subtracting (i) from (iv), we get: B = 31
B’s weight = 31 kg