The average weight of the 8 members of a rowing crew is 72 kg. Two members weighing 78 kg and 66 kg leave the crew, and two newcomers weighing 82 kg and 78 kg take their places. What is the new average weight of the crew?

The crew still has 8 members, so only the total moves. Out go 78 + 66 = 144 kg and in come 82 + 78 = 160 kg, a gain of 16 kg. The old total was $8\times 72=576$ kg, the new total is 592 kg, and 592/8 = 74 kg. 72 assumes that swapping two for two must leave the average alone, but the pair coming in is heavier than the pair going out. 80 is the average of the two newcomers on their own, which ignores the six members who never moved. 88 adds the whole 16 kg gain to the average instead of sharing it among the 8 members.