The average weight of P, Q and R is 45 kg. If the average weight of P and Q is 40 kg and that of Q and R is 43 kg, what is the weight of Q?

31

Total weight of P, Q and R = 135kg 

Total weight of P and Q = 80kg

Total weight of Q and R = 86kg

Weight of R = 135 - 80 = 55kg [ P + Q + R - (P + Q)]

Weight of Q = 86 - 55 = 31kg [ Q + R - (R)]