在一个有 42 名儿童的班级里，儿童的平均体重是 37.5 kg。班里女生的平均体重是 36.7 kg。班里男生的平均体重是 38.8 kg。班里的女生比男生多多少人？ 答：___

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 = $26 - 16 = 10$