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Two pipes A and B can fill a cistern in 15 minutes and 30 minutes respectively. Both pipes are opened together, but after 5 minute pipe B is turned off. The cistern will be full in total :

Solution

✅ Correct Option: 4

Combined rate =115+130=110= \frac{1}{15} + \frac{1}{30} = \frac{1}{10} per minute.

In 5 minutes, half the tank is filled.

A alone fills remaining half in 1/21/15=7.5\frac{1/2}{1/15} = 7.5 min.

Total =5+7.5=12.5= 5 + 7.5 = 12.5 min.

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