The average weight of the eight rowers in a boat goes up by 1.5 kg when a new rower replaces one who weighed 62 kg. The new rower's weight (in kg) is:

A rise of 1.5 kg in the average of eight people means the total went up by $8\times 1.5=12$ kg, and that whole increase comes from the swap, so the newcomer weighs 62 + 12 = 74 kg. Adding the 1.5 kg directly to 62 gives 63.5 kg and forgets that one extra kilogram in the total lifts the average by only 1/8 kg.