Two pipes, P ans Q can fill a cistern in 12 and 15 minutes respectively. Both are opened together, but at the end of 3 minutes the first is turned off. How much longer will the cistern take to fill?

Q: Two pipes, P ans Q can fill a cistern in 12 and 15 minutes respectively. Both are opened together, but at the end of 3 minutes the first is turned off. How much longer will the cistern take to fill?

12(1 - t/25) = 3 ∴ t = 45/4 = 111/4 minutes ∴ Required answer = 111/4 - 3 = 81/4 minutes

81/4 minutes
11 1/4 minutes
73/4 minutes
83/4 minutes

Correct Answer: 81/4 minutes

Explanation:

12(1 - t/25) = 3
∴ t = 45/4 = 11¹/₄ minutes
∴ Required answer = 11¹/₄ - 3
= 8¹/₄ minutes

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