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?

Explanation:-

1st 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
$$\eqalign{ & = \frac{1}{{15}} - \frac{1}{{20}} \cr & = \frac{{4 - 3}}{6} \cr & = \frac{1}{{60}} \cr}$$
Thus, A can complete the work in = 60 days.

2nd 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.