Calculation: Let total work be 60x units (LCM of 10, 15, 60) ⇒ Efficiency of A = 6x units/day ⇒ Efficiency of B = 4x units/day ⇒ Efficiency of C = x units/day ⇒ Work done in 5 days by A & B = (6x + 4x) × 5 = 50x units ⇒ Remaining work = 60x – 50x = 10x units This work is completed by B & C ⇒ Time taken to complete the remaining work = 10x/ (4x + x) = 10x/5x = 2 days ⇒ Total time taken to complete the work = 5 + 2 = 7 days