Two pipes can fill a tank in 15 hours and 4 hours, respectively, while a third pipe can empty it in 12 hours. How long (in hours) will it take to fill the empty tank if all the three pipes are opened simultaneously?

A

$$ \frac{50}{7}$$

B

$$\frac{15}{7}$$

C

$$\frac{30}{7}$$

D

$$\frac{20}{7}$$