AoPS Wiki talk:Problem of the Day/July 3, 2011

Problem

AoPSWiki:Problem of the Day/July 3, 2011

Solution

If $x$ and $y$ vary inversely, then $xy = k$ where $k$ is a constant. We are given $x = 5$ and $y = 8$, so $k = 40$. Now, we want to find $x$ when $y = 10$, so we divide $40$ by $10$ to get $\boxed{4}$.