#### Two pipes were installed in a water tank. One pipe serves as an inlet pipe while another, a multi-purpose pipe, serves as either inlet pipe or a drain pipe. When both pipes are used to fill up the tank, they can fill 7/9 of the tank in 4 hours. In filling the tank, the operator sometimes pushes the wrong button and the multi-purpose pipe functions as a drain pipe. In such cases, only 1/6 of the tank can be filled up in 6 hours. How long will it take the inlet pipe to fill the tank alone?

**SOLUTION:**

Let x be the number of hours the inlet pipe can fill the tank alone, and y be the number of hours the multi-purpose pipe can either fill or drain the tank alone.

The equations areĀ

From equation 1

Substitute 1/x to the second equation.

Solve for x

Therefore, the inlet pipe can fill the tank alone in 9 hours.