Correct Answer - Option 1 : 72 days
∵ A left before 5 days due to which B had to work for 18 days
⇒ 5-day work of A and B together = 18 days work of B
Let the efficiency of A and B be x and y respectively
5 (x + y) = 18y
⇒ 5x + 5y = 18y
⇒ 5x = 13y
⇒ x : y = 13 : 5
Total work = (13 + 5) × 20 = 360 units
∴ Time taken by B alone to complete the work = 360/5 = 72 days