Set up an equation to solve.
$8.50x (since it is the rate of change) + $12 (flat fee) = total money earned (or y). In this case, we have the total, amount, and are looking for x. Plug $139.50 into the equation.
$8.50x + $12 = $139.50
Now, solve for x.
$8.50x + $12-12 = $139.50-12 > <em>$8.50x = $127.5</em>
$8.50/8.50x = $127.5/8.50 > <em>x = 15</em>
So, the correct answer is <u>D. Olivia babysat for 15 hours.</u>
