A man can do a piece of work in 30 hours. If he works with his son then the same piece of work is finished in 20 hours. If the son works alone then he do the work in ?

Answer: Option A

L.C.M. of Total Work = 60
One hours work of M = $$\frac{60}{30}$$ = 2 unit/hours
One hours work of M + S = $$\frac{60}{20}$$ = 3 unit/hours
$$\begin{array}{rl}& \text{Efficiency of S}\\ & =3-2\\ & =1\\ & \text{Required time}\\ & =\frac{60}{1}\\ & \text{ = }60\text{ hours }\end{array}$$