A full tank has an outlet that can empty it in 10 hours and an inlet that can fill it in 15 hours. If both are opened at the same time, how long will the tank take to become empty?

5 hoursCorrect answerYour answer6 hoursCorrect answerYour answer25 hoursCorrect answerYour answer30 hoursCorrect answerYour answer

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CorrectNot quiteAnswer D

Correct answer: D

Emptying beats filling: net emptying rate = $\frac{1}{10} - \frac{1}{15} = \frac{3-2}{30} = \frac{1}{30}$ per hour.
The tank empties in 30 hours.

Worth noting

6 hours adds both rates as if both were emptying. 5 hours subtracts the times. Check: 30 hours is slower than the outlet alone, as it should be.

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