A and B together complete a work in 12 days. A alone works 8 days and B completes the remaining work in 20 days by doing it alone. In how much time will A alone finish the whole work?

18 days

According to the question if A does not work 4 days, B has to work 8 days extra to finish the work".

So,

⇒ 4 days of A's work = 8 days of B's work

Ratio of efficiency = A : B = 2 : 1

Total Work = 12 x 3 = 36

A alone will complete the work in = $\frac{36}{2}$ = 18 days