A can finish a work in 15 days, B can finish the same work in 25 days. They work together for 5 days. The rest of the work is finished by A and C in 4 days. Then C alone can finish the work in:

A+B's 5 days work = 5(1/15+1/25) = 5(5/75+3/75) = 5×8/75 = 8/15. Remaining = 7/15. A+C finish 7/15 in 4 days, so (A+C)'s rate = 7/60 per day. A's rate = 1/15 = 4/60. C's rate = 7/60−4/60 = 3/60 = 1/20. C alone takes 20 days. Hmm, that's 20 days (c). Let me re-check: A+B per day = 1/15+1/25 = 8/75. In 5 days = 40/75 = 8/15. Remaining = 7/15. A+C do 7/15 in 4 days → A+C rate = 7/60. C rate = 7/60 − 1/15 = 7/60 − 4/60 = 3/60 = 1/20. C alone = 20 days.