The ratio of efficiencies of A, B and C is 3 : 7 : 5. Working together they can finish the work in 15 days. A and B work together 10 days and rest of the work is completed by C in how much days?

25 days

Total work = Efficiency x No. of days 

                = 15 x 15 = 225

Part of work completed by A and B in 10 days = 10 x 10 = 100

Work left = 225 - 100 = 125

Number of days taken by C = \(\frac{Total\; work}{Efficiency}\)

                                        = \(\frac{125}{5}\) = 25 days