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?

Answer: Option A

1^st method:
A and B complete a work in = 15 days
One day's work of (A + B) = $$\frac{1}{15}$$
B complete the work in = 20 days;
One day's work of B = $$\frac{1}{20}$$
Then, A's one day's work
$$\begin{array}{rl}& =\frac{1}{15}-\frac{1}{20}\\ & =\frac{4-3}{6}\\ & =\frac{1}{60}\end{array}$$
Thus, A can complete the work in = 60 days.

2^nd method:
(A + B)'s one day's % work = $$\frac{100}{15}$$ = 6.66%
B's one day's % work = $$\frac{100}{20}$$ = 5%
A's one day's % work = 6.66 - 5 = 1.66%
Thus, A need = $$\frac{100}{1.66}$$ = 60 days to complete the work.