#### Answer

We can tell whether or not a proportion is true by cross-multiplying and checking for equality.

#### Work Step by Step

We can tell whether or not a proportion is true by cross-multiplying and checking for equality.
For example, consider this proportion from the previous exercise:
$\displaystyle \frac{30}{6}=\frac{45}{9}$
We cross-multiply:
$30*9=45*6$
And check the equality:
$270=270$
Since we got an identity, we know that the original proportion was true.