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Mathematical Puzzles Hard 3 min

A tank can be filled by a pipe in 6 hours and emptied by another pipe in 9 hours. If both pipes are opened together, how long will the empty tank take to fill?

Hint
Subtract the emptying rate from the filling rate.

Answer

18 hours.

Explanation

The filling rate is 1/6 tank per hour and the draining rate is 1/9. Net rate = 1/18.

Step-by-Step Solution

Net rate = 1/6 - 1/9.
= 3/18 - 2/18.
= 1/18.
Therefore time = 18 hours.
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