The average weight of the 9 members of a school kabaddi team falls by 1.5 kg when a member weighing 78 kg is replaced by a new player. What is the new player's weight?

The team size does not change, so a fall of 1.5 kg in the average means the total fell by $9\times 1.5=13.5$ kg. That whole drop is down to the swap, so the new player weighs 78 - 13.5 = 64.5 kg. 76.5 subtracts the 1.5 kg once, forgetting that the average of NINE people moves only if the total moves nine times as much. 91.5 adds the 13.5 kg, but a lighter player is needed to pull the average down. 66 uses 8 members instead of 9.