Pipe A can fill an empty cistern in 10 minutes, and pipe B can empty the full cistern in 15 minutes. The cistern is empty and both pipes are opened at the same time. In how many minutes will the cistern be full?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 30 minutes
When a filling pipe and an emptying pipe are open together, the cistern gains the difference of their one-minute work.
A fills \(\frac{1}{10}\) and B empties \(\frac{1}{15}\) of the cistern in one minute
Net part filled in one minute \(\displaystyle =\frac{1}{10}-\frac{1}{15}=\frac{3-2}{30}=\frac{1}{30}\)
Time to fill the cistern \(=30\) minutes