30 boys are engaged to do a piece of work in 15 days but it is found that in 5 days, 3/5 work is completed. How many boys may be allowed to go off, so that the work may be finished on time?

20

The correct answer is Option (1) → 20

Each of the 30 boys works for 5 days, so total work done = $30 \times 5 = 150$ boy-days

This 150 boy-days is $\frac{3}{5}$ of the total work, so total work = $\frac{150 \times 5}{3} = 250$ boy-days

Remaining work = $250 - 150 = 100$ boy-days

Remaining days = $15 - 5 = 10$

Required boys = $\frac{100}{10} = 10$ boys

Number of boys who may be allowed to go off = $30 - 10 = 20$