Explanation:

- Pipe A fills the cistern in 8 hours, so in one hour, it fills 1/8 of the cistern.

- Pipe B fills the cistern in 10 hours, so in one hour, it fills 1/10 of the cistern.

- Both pipes run for 2 hours:

- In 2 hours, both pipes together fill: $2\times (\frac{1}{8}+\frac{1}{10})=2\times \frac{9}{40}=\frac{9}{20}$ of the cistern.

- The remaining cistern $(1-\frac{9}{20})$ is $\frac{11}{20}$.

- Pipe A continues with an outlet pipe that drains in 20 hours:

- Pipe A fills $\frac{1}{8}$ per hour. Outlet drains $\frac{1}{20}$ per hour.

- The net fill rate is: $\frac{1}{8}-\frac{1}{20}=\frac{5}{40}-\frac{2}{40}=\frac{3}{40}$ per hour.

- Time to fill the remaining $\frac{11}{20}$ at $\frac{3}{40}$ per hour:

- Time = $\frac{\frac{11}{20}}{\frac{3}{40}}$ = $\frac{11}{20}\times \frac{40}{3}=\frac{22}{3}$ hours = 7 hours 20 minutes.

- Option 3: 7 hours 20 minutes is correct.

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