M alone can complete a work in 50 days and N alone can complete the same work  in 100 days . If both of them work together, then in 25 days, what percent of the total work will be completed?

75%

Total work = 100

Efficiency of M and N = 3

Work will be completed by M and N together in = $\frac{Total\;work}{efficiency}$

                                           = $\frac{100}{3}$

Amount of work done in 25 days = 25 x work done in 1 day

                                                = 25 x $\frac{3}{100}$ = 75

Percentage of work completed = $\frac{75}{100}$ x 100 = 75%