A can do a piece of work in 20 days and B in 15 days. With help of C, they finish the work in 5 days. In how many days C alone can do the same work ?

Answer: Option D

L.C.M. of Total Work =60
One day work of A = $$\frac{60}{20}$$ = 3 unit/day
One day work of B = $$\frac{60}{15}$$ = 4 unit/day
One day work of A + B + C = $$\frac{60}{5}$$ = 12 unit/day
$$\begin{array}{rl}& \text{C’s efficiency}\\ & =12-3-4\\ & =5\\ & \text{C will complete total work in}\\ & =\frac{60}{5}\\ & \text{ = 12 days}\end{array}$$