A, B and C are accountants. A alone can file a return in 6 hours and B alone can file the same return in 10 hours. All three together can do it in 3 hours. In what time will C alone file the same return?

15 hours

Let the total work = LCM of 3, 6, 10 = 30

Efficiency of C = eff(A, B, C) - eff(A) - eff(B)

                      = 10 - 5 - 3 = 2

Time taken C alone to complete the work = $\frac{total\;work}{efficiency of C}$ = $\frac{30}{2}$  = 15 hours