Answer:
15
Explanation:
For 1 house, the charge just to go to a house is $12.
In each house you made $57.
$57 - $12 = $45
The charge per house just for raking was $45.
You charge $9/hour.
$45/($9/hr) = 5 hours
You shoveled 5 hours in each house.
3 * 5 hours = 15 hours
Answer: 15 hours