Since the photo booth charges a $500 fee for two hours at the party and an additional 50 dollars per hour, we have:
50x+500=C
Where x is the hours stayed and C is the total cost. Since Cindy doesn't want to spend more than 700 dollars, we have:
![image](https://img.qammunity.org/2019/formulas/mathematics/college/d6vgupfet1b6qywfcjbo3l20zt69o9z60p.png)
So she spent an additional 4 hours on the rental.