Final answer:
Two keys together can fill the tank in 3 hours.
Explanation:
Llave 1 takes 6 hours to fill the tank, while llave 2 takes 9 hours. Combined, they fill the tank in 3 hours. To find this, you can use the formula: 1/x = 1/6 + 1/9. Solving for x, you get x = 3 hours.
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