#### Answer

A proportion sets two rates/fractions/ratios equal to each other.
We can visualize the cross-multiplication as two arrows: one from 30 to 9 and the other from 6 to 45.

#### Work Step by Step

A proportion sets two rates/fractions/ratios equal to each other.
For example, one might say that Jim is twice as tall as Bill. If Jim is 6 feet tall, then we can set up a proportion like so:
$\frac{6}{x}=\frac{2}{1}$
This can be used to find $x$, Bill's height by cross-multiplication:
$6*1=2*x$
$x=3$
In the fraction provided, we have:
$\displaystyle \frac{30}{6}=\frac{45}{9}$
We can visualize the cross-multiplication as two arrows: one from 30 to 9 and the other from 6 to 45. The numbers multiply thus:
$30*9=45*6$
$270=270$
This method can be used to solve for any variables (as was done in the above example).