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 ?

A. 5 days

B. 6 days

C. 10 days

D. 12 days

Answer: Option D

Solution(By Examveda Team)

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
$$\eqalign{ & {\text{C's efficiency}} \cr & = 12 - 3 - 4 \cr & = 5 \cr & {\text{C will complete total work in}} \cr & = \frac{{60}}{5} \cr & {\text{ = 12 days}} \cr} $$