Correct Answer - Option 3 : 30

**Given :**

An inlet pipe can fill a tank in 20 hours and an outlet pipe can empty the filled tank 50 hours

**Concept used :**

Work efficiency = Total work/Total working time

**Calculations :**

Total work = LCM of 20 and 50 = 100 units

Inlet pipe per hour work = 100/20 = 5 units

Outlet pipe per hour work = 100/50 = 2 units

Total work of both pipes = 5 - 2 = 3 units (outlet pipe works opposite to inlet pipe)

Time required to fill 75% filled pipe = 75/3 = 25 hours

Now time taken by inlet pipe to fill the remaining tank = 25/5 = 5 hours (outlet pipe ios closed after 75% pipe is filled)

Total time = 25 + 5 = 30 hours

**∴ Option 3 will be the correct answer.**