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Water from the first faucet fills the swimming pool in $20$ hours. Water from each of the three other faucets fills the same swimming pool $3$ 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 $\frac1{20}$ and that of the other two is $\frac3{20}$.  Thus it takes $1\div (\frac1{20}+\frac3{20}\times3)=2$ hours to fill the pool. 