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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?

1. 5 min
2. 6 min
3. 7 min
4. 8 min

Correct answer: 6 min

Solution

Initially, A fills $1/18$ tank per minute and B fills $1/36$ per minute, so together they fill $1/12$ tank per minute. In 6 minutes they fill $6/12 = 1/2$ tank. After the change, A works at $1/36$ and B at $1/18$, so the combined rate is again $1/12$ tank per minute; thus the remaining half takes 6 more minutes.

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