There are 50 students in a class, of which 40 are boys. The average weight of the class is 44 kg and that the girls is 40 kg. Find the average weight of the boys.
Given,
Average weight of 50 students in class = 44 kg
Total weight of 50 students = 50 × 44 = 2200 kg
Number of girls = 50 – number of boys = 50 – 40 = 10
Average weight of 10 girls = 40 kg
Total weight of 10 girls = 10 × 40 = 400 kg
Hence,
Total weight of 40 boys in class = total weight of class – weight of girls
= 2200 – 400 = 1800 kg
Average weight of boys = 
Hence, Average weight of 40 boys = 45 kg.
20