A and B together can do a piece of work in 12 days, while B alone can finish it in 30 days. A alone can finish the work in :
[A]15 days
[B]18 days
[C]20 days
[D]25 days

20 days
A’s 1 day’s work = \frac{1}{12}-\frac{1}{30} = \frac{5-2}{60} = \frac{3}{60} = \frac{1}{20}
So, A alone will complete the work in 20 days.
Hence option [C] is correct answer.