A is twice as good as B and together they finish a piece of work in 16 days. The number of days taken by A alone to finish the work is = ?

$$\eqalign{ & {\text{B}}:{\text{A}} \cr & \,1:2 \to {\text{Efficiency ratio}} \cr & {\text{Total work}} = 16 \times \left( {1 + 2} \right) = 48 \cr} $$
Number of days taken by A to complete the work
$$\eqalign{ & = \frac{{48}}{2} \cr & = 24{\text{ days}} \cr} $$