One pipe can fill a tank in four hours; another pipe can fill the same tank in three hours. How long would it take to fill the tank if both pipes filled it at the same time? [?] hours Enter the number that belongs in the green box. Enter

9514 1404 393

Step-by-step explanation:

The first pipe fills 1/4 of the tank in 1 hour. The second pipe fills 1/3 of the tank in 1 hour. So, in 1 hour, the amount of the tank that is filled is ...

This number is the fill rate in tanks per hour. We want the reciprocal of that, the fill time in hours per tank. The reciprocal of 7/12 is 12/7.

It will take 12/7 hours to fill the tank.

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Comment on the answer

It looks like you may be asked to fill two "green boxes" to provide the complete answer. The box shown as green in this question is the numerator of the fraction, so should be filled with 12. (We expect that the denominator entry will be made available as soon as you enter the numerator. Your experience with this web site will tell you if that is true.)