ThinkTank #5 - Maths
This is a test on the student's conceptual understanding of Average
Since the average mass of the group increased by 2kg when John joined the group, this meant that John's extra mass would have been evenly distributed to all the children in the group at first.
Thus, we would come to the following equation: (46.5 kg - 38.5 kg) ÷ X = 2kg
Solving for it would show us that there were 4 children in the group at first!