#### Explanation:

25% is the same as $\frac{1}{4}$

If The amount that Mamie has saved is $\frac{1}{4}$ of what she needs, then the full amount will be 4 times as much.

$161.25 xx 4 =$645

Or you can write a proportion:
"If $161.25 is 25%, how much is 100%?" $\frac{25}{100} = \frac{161.25}{x}$$x = 161.25 \times \frac{100}{25}$x =$645