One way to begin is to compare everyone with the new average of 42 kg. The original group is short by 3 kg per boy on average. Arnold brings 24 kg above 42 kg. His excess balances eight shortfalls of 3 kg, so there were 8 boys originally.
No individual boy is assumed to weigh 39 kg. The useful relationship is between the group’s average shortfall and Arnold’s excess. A student may also solve the question with an equation; seeing this relationship gives the numbers meaning and offers another way in.

