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Water from the first faucet fills the swimming pool in $36$ hours. Water from each of the two other faucets fills the same swimming pool $4$ times faster. In how many hours will the swimming pool be filled if all three faucets are opened? 
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The efficiency of the first faucet is $\frac1{36}$ and that of the other two is $\frac4{36}$.  Thus it takes $1\div (\frac1{36}+\frac4{36}\times2)=4$ hours to fill the pool. 