M and N can do a piece of work in 30 days, while N and O can do the same work in 24 days and O and M in 20 days. They all work together for 10 days till N and O leave. How many days more will M take to finish the work?

Let M, N, O's individual work rates be m, n, o work/day. M+N: 30 days, so m+n = 1/30. N+O: 24 days, so n+o = 1/24. O+M: 20 days, so o+m = 1/20. Adding all three: 2(m+n+o) = 1/30 + 1/24 + 1/20 = 1/8. So m+n+o = 1/16. In 10 days together, they complete 10/16 = 5/8 of work. Remaining work = 3/8. M's rate alone = (m+n+o) - (n+o) = 1/16 - 1/24 = 1/48. M needs (3/8)/(1/48) = 18 days.