A and B can together do a piece of work it in 6 days and A alone can do it in 9 days. The number of days B will take to do it alone is ?

Answer: Option A

L.C.M. of Total Work = 36
One day work of A + B = $$\frac{36}{6}$$ = 6 unit/day
One day work of A = $$\frac{36}{9}$$ = 4 unit/day
$$\begin{array}{rl}& \text{B’s efficiency}\\ & =6-4\\ & =2\\ & \text{Time taken by B}\\ & =\frac{36}{2}\\ & =18\text{ days}\end{array}$$