Moderate Time and Work Solved QuestionAptitude Discussion

 Q. A man can do a piece of work in 60 hours. If he takes his son with him and both work together then the work is finished in 40 hours. How many hours will the son take to do the same job, if he worked alone on the job?
 ✖ A. 20 ✖ B. 60 ✖ C. 100 ✔ D. 120

Solution:
Option(D) is correct

If the man takes 60 hours to complete the work, then he will finish $\left(\dfrac{1}{60}\right)^{th}$of the work in 1 hour.

Let us assume that his son takes $x$ hours to finish the same work.

If they work together for 1 hour they will finish $\dfrac{1}{60}+\dfrac{1}{x}=\dfrac{1}{40}$of the work.

Therefore,

$\dfrac{1}{x}=\dfrac{1}{20}$

The son, working alone would take 120 hours to complete the work.