Time taken by A alone to complete the entire work = 10/0.5 = 20 days Time taken by B alone to complete the entire work = 20 + 10 = 30 days Let the total work = L.C.M of 20, 30 and 12 = 420 units Then, efficiency of B = (420/28) = 15 units/day Combined efficiency of B and C = (420/12) = 35 units/day So, efficiency of C alone = 35 – 15 = 20 units/day So, time taken by C alone to complete the entire work = (420/20) = 21 days