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[distant stirrup]

Filling the pool with the first pipe takes 9 hours longer than with the first and second pipes together and 7 hours less than with the second pipe only. In how many hours does the pool fill with both pipes together?

[clear dome]

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[noble mortar]

Let the time required to fill the pool using the first pipe alone be $t$ hours. Then
$$\frac{1}{\frac{1}{t}+\frac{1}{t+7}}=t-9$$

[dense patrol]

ℝafain

[distant stirrup]

So t will be 21 and how to calculate both work time?

[noble mortar]

Look at what t means again, and how it is related to the time taken to fill the pool with both pipes

[distant stirrup]

.close