The average age of 125 students in a group is 16.2 years. 40% of the students are boys and the rest are girls. The average age of the boys is 20% more than the average age of the girls. What is the average age (in years) of the boys?

Let the average age of girls be g years. Then the average age of boys is 1.2g (20% more). With 125 students (50 boys, 75 girls), total age = 50(1.2g) + 75g = 135g. The overall average is 135g/125 = 16.2, giving g = 15. Therefore, boys' average = 1.2 × 15 = 18 years. This uses the weighted average principle combined with the percentage relationship between the two averages.