A, B and C can finish a job working alone in 20, 30 and 60 days respectively. They all work together for 1 day, then A and B quit. How many days C working alone will take to finish the remaining part of the job?

Work rates: A = 1/20, B = 1/30, C = 1/60 per day. Combined for 1 day: 1/20 + 1/30 + 1/60 = 3/60 + 2/60 + 1/60 = 6/60 = 1/10. Work done = 1/10, remaining = 9/10. C's rate = 1/60, so C needs (9/10) ÷ (1/60) = 9/10 × 60 = 54 days. Option 60 ignores the first day's work, 6 is the error of taking 1/10 directly, 27 is a miscalculation.