What is the ratio of 9 to 3?

$3 : 1$
In order to simplify a ratio we need to find a common factor. In this case, both $3$ and $9$ are divisible by $3$, therefore we can divide both sides to simplify the ratio:
$9 : 3 = \frac{9}{3} : \frac{3}{3} = 3 : 1$