A can finish a piece of work in 18 days and B can do the same work in half of the time taken by A. Then working together what part of the same work they can finish in a day ?

Answer: Option A

So let's solve now,
A's 1 day work = $$\frac{1}{18}$$
B's 1 day work = $$\frac{1}{9}$$ [because B take half time than A]
(A + B)'s one day work
$$\begin{array}{rl}& =\frac{1}{18}+\frac{1}{9}\\ & =\frac{1+2}{18}\\ & =\frac{1}{6}\end{array}$$