The two inlet pipes can fill the tank in 4 hours and 10 hours.
The work done by the inlet pipes per hour is calculated to be:
[tex]\Rightarrow\frac{1}{4}+\frac{1}{10}=\frac{7}{20}\text{ units per hour}[/tex]The outlet pipe drains the tank in 2 hours. The work done will be:
[tex]\Rightarrow\frac{1}{2}\text{ units per hour}[/tex]Therefore, if both inlet pipes and the outlet pipes are opened at the same time, the work done by the pipes combined will be:
[tex]\Rightarrow\frac{7}{20}-\frac{1}{2}=-\frac{3}{20}\text{ units per hour}[/tex]Since the work done to fill the tank is negative, it means that the tank will never get full as it empties faster than it fills up.
