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A certain tank can be filled by pipe A in 12 minutes. Pipe B can empty the tank in 18 minutes.
If both pipes are open, how long will it take to fill or empty the tank?
It takes less time to fill it than to empty it, therefore with both valves open the tank will be filled eventually
Let t = time to fill the tank with both valves open
Let a full tank = 1
multiply by 36
Cancel the denominators, results
3t - 2t = 36
t = 36 hrs to fill the tank with both valves left open