Explanation

The average weight of A, B, and C is 45 kg, so the total weight is 45 x 3 = 135 kg.

The average weight of A and B is 40 kg, so their total weight is 40 x 2 = 80 kg.

The average weight of B and C is 43 kg, so their total weight is 43 x 2 = 86 kg.

Now, let's find the weight of B:

Subtract the weight of A from the total weight of A and B: 80 kg - A = B

Subtract the weight of C from the total weight of B and C: 86 kg - C = B

Since the total weight of A, B, and C is 135 kg, we can set up the equation:

A + B + C = 135

Substituting the expressions for B from steps 1 and 2, we get:

(80 kg - B) + B + (86 kg - B) = 135

Simplifying the equation, we get:

166 kg - B = 135

Subtracting 166 kg from both sides:

-31 kg = -B

Dividing both sides by -1:

B = 31 kg

Therefore, the weight of B is 31 kg.