In a class of 42 children, the average mass of the children was 37.5 kg. The average mass of the girls in the class was 36.7 kg. The average mass of the boys in the class was 38.8 kg. How many more girls than boys were there in the class? Ans: ___

1. Calculate the total mass of all children

Total mass = $42 \times 37.5 = 1575$ kg

2. Use the assumption method to find the number of boys

Assume all 42 children are girls.
Total mass would be $42 \times 36.7 = 1541.4$ kg.
Difference from actual total mass = $1575 - 1541.4 = 33.6$ kg.
Difference in average mass between a boy and a girl = $38.8 - 36.7 = 2.1$ kg.
Number of boys = $33.6 \div 2.1 = 16$

3. Find the number of girls and the difference

Number of girls = $42 - 16 = 26$
Difference = $26 - 16 = 10$