The average weight of the girls in a group is 15 kg and the average weight of the boys is 30 kg. The average weight of all the boys and girls together is 25 kg. If there are 12 boys, how many girls are there?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 6
By the rule of alligation, the numbers in the two groups are in the ratio of the distances of the other group’s average from the overall average.
Boys \(\propto 25-15=10\)
Girls \(\propto 30-25=5\)
Boys : Girls \(=10:5=2:1\)
Girls \(\displaystyle =\frac{12}{2}=6\)
Check: \(12\times30+6\times15=450\) and \(450\div18=25\) kg. There are 6 girls.