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.