A can complete a work in 20 days and B in 30 days. A worked alone for 4 days and then B complete the remaining work alone with C in 18 days. In how many days can C working alone complete the work?

Work done by A in 4 days = 4/20 = 1/5
∴ Remaining work = 1 - 1/5 = 4/5
Let C can working alone complete the work in N days .
According to the question,
18/30 + 18/N = 4/5
⇒ 18(N + 30) x 5 = 4 x 30 x N
⇒ 90N + 2700 = 120N
⇒ 30N = 2700
∴ N = 90 days