A can do a piece of work in 15 days, which B can do it in 10 days. B worked at it for 8 days. A can finish the remaining work in?

$$\eqalign{ & {\text{B's 8 day's work}} \cr & = \left( {\frac{1}{{10}} \times 8} \right) \cr & = \frac{4}{5} \cr & {\text{Remaining work}} \cr & = \left( {1 - \frac{4}{5}} \right) \cr & = \frac{1}{5} \cr & {\text{Now,}} \cr & \frac{1}{{15}}{\text{work is done by A in 1 day}}{\text{.}} \cr & \therefore \frac{1}{5}{\text{ work is done by A in}} \cr & = \left( {15 \times \frac{1}{5}} \right) \cr & = 3{\text{ days}}{\text{.}} \cr} $$