In a class of 39 students there are 26 girls with average weight 42 kg. If the average weight of the whole class is 48 kg, then what is the average weight of the boys in the class?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 60 kg
An average times the number of people gives the total weight. The boys’ total is the class total less the girls’ total.
Total weight of the class \(=39\times48=1872\) kg
Total weight of the 26 girls \(=26\times42=1092\) kg
Boys: \(39-26=13\), with total weight \(1872-1092=780\) kg
Average weight of the boys \(=780\div13=60\) kg
Shortcut: Work with the differences from the class average of 48 kg; what the girls lack, the boys must make up.
Each girl is \(48-42=6\) kg below the average: the girls together are \(26\times6=156\) kg short
The 13 boys make up the 156 kg: each is \(156\div13=12\) kg above the average
Average weight of the boys \(=48+12=60\) kg