A is thrice as efficient as B. A started working and after 4 days he was replaced by B. B then worked for 15 days and left. If A and B together finished 75% of the total work, in how many days B alone can finish the whole work?

A is thrice as efficient as B, so if A does 3 units of work per day, B does 1 unit per day. A worked for 4 days: work done = 4×3 = 12 units. B worked for 15 days: work done = 15×1 = 15 units. Total work done = 12+15 = 27 units. This represents 75% of total work, so 0.75 × Total work = 27. Total work = 27/0.75 = 36 units. B alone does 1 unit per day, so B would take 36 days to complete the whole work alone.