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Water from the first faucet fills the swimming pool in $70$ minutes. Water from each of the two other faucets fills the same swimming pool $3$ times faster. In how many minutes will the swimming pool be filled if all three faucets are opened? 
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The efficiency of the first faucet is $\frac1{70}$ and that of the other two is $\frac3{70}$.  Thus, it takes $1\div (\frac1{70}+\frac3{70}\times2)=10$ minutes to fill the pool. 