#### Answer

800 dollars must be invested to raise earnings to 72 dollars per year.

#### Work Step by Step

Let X represent the amount of money that must be invested to raise the earnings to 72 per year.
Now, let’s set up a proportion in which one ratio compares the invested amounts and the other ratio compares the corresponding earnings.
So, $\frac{500}{X}$ = $\frac{45}{72}$
To solve this equation, we equate the cross products.
500 $\times$ 72 = X $\times$ 45
36000 = 45X
Divide both sides by 45.
X = 800.
800 dollars must be invested to raise earnings to 72 dollars per year.