The average age of boys in a school with 560 students is 12 years and that of girls is 11 years. If the average age of a school is 11 years 9 months, then what will be the number of girls in the school?

Correct option is D

Given
Total students = 560
Average age of boys = 12 years
Average age of girls = 11 years
Average age of school = 11 years 9 months = 11.75 years

Concept Used
Mixture and Allegation

Solution
Ratio of Boys : Girls = (Mean - Girls Avg) : (Boys Avg - Mean)
Ratio = (11.75 - 11) : (12 - 11.75)
Ratio = 0.75 : 0.25
Ratio = 3 : 1
Total parts = 3 + 1 = 4
Number of girls = $$\frac{1}{4} \times 560$$​​
Number of girls = 140

Final Answer
So the correct answer is (d)