A can complete a piece of work in 18 days, B in 20 days and C in 30 days. B and C together start the work and are forced to leave after 2 days. The time taken by A alone to complete the remaining work is

Amount of work done by B and C together in unit time will be $$\frac{1}{20}+\frac{1}{30}$$=$$\frac{5}{60}$$

hence in two days amount of work done by B and C together = $$\frac{10}{60}$$
                                                                                         =$$\frac{1}{6}$$
remaining work = $$\frac{5}{6}$$

A does $$\frac{1}{18}$$ amount of work in unit time

hence  $$\frac{5}{6}$$ amount of work will be done in 15 days.