Answer:
At least 9 hours.
Explanation:
Dan already has $400 from the car-wash
So our equation is
400 + 15x > 520
X = hours of dog walking
First, you need to isolate 15x
So
400 + 15x > 520
-400 -400
15x > 120
Then you divide both sides by 15, so
X > 8
Dan has to work at least 9 hours of dog walking to get more than 520