A tank is connected with both inlet pipes and outlet pipes. Individually, an inlet pipe can fill the tank in 7 hours and an outlet pipe can empty it in 5 hours. If all the pipes are kept open, it takes exactly 7 hours for a completely filled-in tank to empty. If the total number of pipes connected to the tank is 11, how many of these are inlet pipes?

- 2
- 4
- 5
- 6

**Solution**

Let there be x inlet and y outlet pipes.

x + y = 11

Assume that the capacity of the tank is 35 units.

Inlet pipe fills 5 units and the outlet pipes empties 7 units in one hour.

The completely filled tank empties in 7 hours.

7y - 5x = 5

Solving two equations, y = 5 and x = 6

**The correct option is D.**