Pipe A can fill an empty tank in 5 hours while pipe B can empty the full tank in 6 hours.

Pipe A can fill a tank in 5 hours.

Pipe B can empty a full tank in 6 hours.

Pipe A fills $\frac{1}{5}$ of the tank in one hour.

Pipe B empties $\frac{1}{6}$ of the tank in one hour.

Part of the tank filled in one hour using both pipes A and B $=\frac{1}{5}-\frac{1}{6}=\frac{6-5}{30}=\frac{1}{30}$

It takes $\frac{30}{1}$ or 30 hours to fill the tank completely.