- Pipe A can fill the pool in 2 hours. So its rate is 1/2.

- Pipe B can fill the pool in 18 hours. So its rate is 1/18.

Working together, Pipe A and Pipe B have a combined rate of 1/2 + 1/18 = 10/18 or 5/9 of a pool per hour. This also means that in 1 hour, both pipes can fill 5/9 of a pool.

Since t = 1/r, we learn that the time taken to fill the pool will be 9/5 hours (1h 48m) when pipes A and B work together.

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