A tap can fill a tank in 3.5 hours. However, due to a leak at the bottom, it takes an additional 30 minutes to fill the tank completely. How long will the leak take to empty the full tank on its own?

Explanation

The tap fills the tank in 3.5 hours, so its filling rate is 1/3.5 = 2/7 tanks per hour. With the leak, the filling time increases to 4 hours, so the net filling rate is 1/4 tanks per hour. Let the leak's emptying rate be 1/x tanks per hour. Then, (2/7) - (1/x) = 1/4. Solving for x gives x = 28 hours, meaning the leak alone empties the tank in 28 hours.