Pipe A can fill a tank in 18 minutes and Pipe B can fill it in 36 minutes. Both are opened together. After 6 minutes, the rate of Pipe A becomes half and Pipe B’s rate becomes double. How many more minutes are required to fill the tank?

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Accepted Answer

1. **Rates:** Pipe A = $1/18$ tank/min, Pipe B = $1/36$ tank/min. Together = $1/12$ tank/min.

2. **First 6 min:** Filled = $6	imes(1/12)=1/2$ tank. So half remains.

3. **After change:** A = $1/36$, B = $1/18$. Together = $1/12$ tank/min again.

4. **Finish time:** Remaining $1/2$ at $1/12$ takes $(1/2)\div(1/12)=6$ min.

FINAL_ANSWER: b

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