A is twice as good a workman as B. If they work together, they can complete a job in 18 days. If A alone does the job, in how many days he will complete the job ?

Answer: Option A

(A's 1 day's work) : (B's 1 day's work)
$$\begin{array}{rl}& =2:1\\ & \left(\text{A}+\text{B}\right)\text{’s 1 day’s work}=\frac{1}{18}\\ & \therefore \text{A’s 1 day’s work}\\ & =\left(\frac{1}{18}\times \frac{2}{3}\right)\\ & =\frac{1}{27}\end{array}$$
Hence, A alone can finish the work in 27 days.