Explanation

Average wait of A, B, and C = 45 kg.

(A+B+C)/3 =  45 kg

A+B+C = 45 *3

so A+B+C = 135 kg  …………….( 1) 

Average weight of A and B= 40 Kg

A+B= 40 x 2 = 80 kg …………………. (2)

The average weight of B and C= 43

B+C= 43  x 2 = 86 kg ……………………(3) 

Let us add 2 & 3  = 80 + 86 kg = 166 kg

But from 1 we know that A+B+C = 135 kg

So, 135+B= 166 kg.

Thus, B = 166 – 135 = 31 kg.