Answer:
24 hours
Step-by-step explanation:
The milk tank can be filled in 8 hours through a valve.
So, in one hour the tank can be filled by [tex]\frac{1}{8}[/tex] part of the tank through this valve.
Again another valve takes 12 hours to empty the tank.
So, in one hour the tank can be emptied by [tex]\frac{1}{12}[/tex] part of the tank through this valve.
Hence, if both the valves are open then within one hour the tank will be filled by [tex](\frac{1}{8} -\frac{1}{12} ) =\frac{1}{24}[/tex] parts.
Therefore, the whole tank will be filled by [tex]\frac{1}{\frac{1}{24} } = 24[/tex] hours. (Answer)