A pipe fills a tank in 8 hours. After 4 hours an outlet that empties the full tank in 16 hours is also opened. How long does the tank take to fill from the start?

Explanation

1. 01

   In 4 hours the pipe fills half the tank.

2. 02

   Then the net rate is $\frac{1}{8} - \frac{1}{16} = \frac{1}{16}$, so the other half takes 8 hours: $4 + 8 = 12$ hours in all.