A tank is filled by one pipe in 6 hours and emptied by another in 12 hours. If both are opened together, how long will the tank take to fill?
Correct answer: B. 12 hours
- A. 8 hours
- B. 12 hours
- C. 6 hours
- D. 18 hours
Explanation
In one hour the filling pipe adds one sixth and the emptying pipe removes one twelfth, giving a net gain of one twelfth per hour, so the tank fills in 12 hours. Working in fractions of the job per unit time is the standard method for pipe and cistern problems. If the outlet were faster than the inlet, the tank would never fill at all.
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