Solution

Time taken by A to complete 100% work = 4 × 5 = 20 days Time taken by B to complete 100% work = 10 × 3 = 30 days L.C.M of 20 and 30 = 60 = Total work Efficiency of A = 60/20 = 3 units/day Efficiency of B = 60/30 = 2 units/day Worked done by A and B together in 9 days = (3 + 2) × 9 ⇒ 45 units Remaining work = (60 – 45) units ⇒ 15 units C can complete the remaining work in 6 days Efficiency of C = 15/6 = 2.5 units/day Total efficiency of B and C = (2 + 2.5) units/day ⇒ 4.5 units/day Now, 75% of total work = 75% of 60 units ⇒ 45 units Required time = 45/4.5 days ⇒ 10 days ∴ B and C can complete 75% work in 10 days