7.5. A steamship from Gorky to Astrakhan takes 5 days, while from Astrakhan to Gorky it takes 7 days. How many days will it take for driftwood to float from Gorky to Astrakhan?
7.5. Answer: 35. Let $v$ be the speed of the river current, $u$ be the speed of the steamship, and $l$ be the distance from Gorky to Astrakhan. According to the problem, $\frac{l}{u+v}=5$ and $\frac{l}{u-v}=7$; we need to find $l / v$. From the system of equations $l=5 u+5 v, l=7 u-7 v$, we find $l / v=35$.