A tank can be filled with water through two pipes. One pipe can fill the tank in 3 hours, and both pipes together can fill the tank in 1 hour and 12 minutes. How many hours does it take to fill the tank if only the second pipe is used?
In 1 hour and 12 minutes, or 1.2 hours, the first pipe fills $\frac{2}{5}$ of the tank, which implies that the second pipe fills $\frac{3}{5}$ of the tank in the same 1.2 hours. Therefore, the entire tank is filled by the second pipe in

$$
\frac{1.2}{3} \cdot 5=2 \text { hours. }
$$