M and N together can do a piece of work in 30 days, N and S together can do it in 20 days. M starts the work and works on it for 5 days, then N takes it up and works for 15 days. Finally S finishes the work in 18 days. In how many days can C do the work when doing it separately?

The question asks about 'C' but the work is done by M, N, and S. Assuming C is a typo for S, we calculate: M does 5/M, N does 15/N, S does 18/S work. Combined, they complete the job: 5/M + 15/N + 18/S = 1. From M+N working together in 30 days and N+S in 20 days, we get 1/M + 1/N = 1/30 and 1/N + 1/S = 1/20. Solving these equations gives S = 24 days alone.