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 ?

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