A steamship traveled from Nizhny Novgorod to Astrakhan in 5 days, and back in 7 days. How many days do rafts float from Nizhny Novgorod to Astrakhan

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Let the distance from Nizhny to Astrakhan be $S$. Then the speed of the steamboat downstream is $S / 5$, and upstream it is $S / 7$. Therefore, the speed of the current (and thus the rafts) is $1 / 2(S / 5 - S / 7) = S / 35$.

## Answer

35 days.