If $x:9::5:y$, then $xy=?$

The given expression is in proportion form:
$$x:9::5:y$$

This can be written mathematically as:

$$\frac{x}{9}=\frac{5}{y}$$

By performing cross-multiplication:

$$x\times y=5\times 9$$

$$xy=45$$

Therefore, the value of $xy$ is $45$.