Tap A can fill an empty tank alone in 6 hours. Tap B alone can empty the same tank, when filled, in 8 hours. If they are opened in alternate hours starting with A, then in how much time will the tank be full for the first time?

41 hours

A+ = 6 hrs, B- = 8 hrs,

⇒ They are opened on alternate hours,

 1st hour = +4 units

 2nd hour = - 3 units,

Every 2 hours, 1 units is filled,

So, in 40 hours, 20 units is filled,

Therefore in 41 hours, 24 units are filled.