Q. A man completes 7/8 of a job in 21 days. How many more days will it take him to finish the job if quantum of work is further increased by 50%

Explanation : 

A man completes 7/8 of a job in 21 days, work increase 50%, New days required ?

Let’s say the total amount of work is W. The man completes 7/8 of the job in 21 days, so his rate of work is (7/8)W/21 = W/24.

Now let’s say the quantum of work is increased by 50%, so the new amount of work to be completed is (1 + 0.5)W = 1.5W.

The remaining amount of work to be completed is 1.5W – (7/8)W = (5/8)W.

Since the man’s rate of work is W/24, it will take him (5/8)W / (W/24) = 15 days to complete the remaining work.

So it will take him an additional 15 days to finish the job if the quantum of work is further increased by 50%, and therefore option (d) “15” is the correct answer.