# Arithmetic Aptitude :: Pipes & Cistern

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## 9. A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If both taps are opened simultaneously, then after how much time will the cistern get filled ?

**Answer:**

**Option C**

**Explanation :**

Time taken by tap A to fill the cistern=4 hrs

so work done by tap A in 1 hour = 1/4th

Time taken by tap B to empty the full cistern = 9 hours

so work done by tap B in 1 hour = 1/9th

=> Work done by (A + B) in 1 hour=(1/4 - 1/9)=5/36

Therefore, the tank will fill the cistern = 36/5 hours=7.2 hours.

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