An inlet pipe A originating from a river can fill a reservoir in 30 days. And an outlet pipe B, which is capable of emptying the completely filled reservoir in 50 days, drains out the water from the reservoir to an irrigation canal. The pipes are opened on alternate days starting with pipe A. On which day from the beginning will the reservoir get completely filled for the first time?

147th

A+ = 30 days, B- = 50 days,

⇒ They are opened on alternate days, therefore,

 1st day = +5 units,

 2nd day = -3 units,

Every 2 days, 2 units is filled,

So, in 144 days, 144 units is filled,

and in 146 days, 146 units is filled, 

Therefore in 147 , 150 units is filled.