## Geometry: Common Core (15th Edition)

$\frac{a}{9} = \frac{b}{10}$
One of the properties of proportions is that we can switch the means, and the resulting proportion would be equivalent to the original proportion. In this exercise, $b$ and $9$ are the means, so we can switch these values: $\frac{a}{9} = \frac{b}{10}$