If A and B together can complete a piece of work in 15 days and B alone in 20 days, in how many days can A alone complete the work ?

B does complete work in 20 days
hence in unit time amount of work done will be $$\frac{1}{20}$$
suppose in unit time amount of work done by A will be $$x$$
hence when they are working together amount of work done in unit time will be 
$$x$$ + $$\frac{1}{20}$$ = $$\frac{1}{15}$$ (given)
$$x$$= $$\frac{1}{60}$$
it means A does $$\frac{1}{60}$$ amount of work in unit time
now unit work will be done ,when A alone is working, in 60 days