Exam Maths Puzzle Numerical Ability Averages

The average age of a class of 60 students is 12 years. If the number of girls is increased by 3/4th and the number of boys is decreased by half, the total number of students does not change; nor does the average age of either the boys or the girls change. But the combined average age changes to 13.5 years. The average age of the boys is
Option
(a) 8 years
(b) 10 years
(c) 12 years
(d) Cannot be determined

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Exam Other Question
If 15 men or 24 women or 36 boys can complete a piece of work in 12 days by working 8 hours a day, then how many men must be required to work with 12 women and 6 boys to complete another piece of work
1
2
4
times the previous piece of work in 30 days by working 6 hours a day?
Option
(a) 6
(b) 4
(c) 8
(d) 10 f(x) and g(x) are two functions defined as:
f(x) = 2x + 3 and g(x) = 3x + 7, then what is the value of g(f(5))?
Option
(a) 51
(b) 42
(c) 46
(d) 37