Chapter 1 - Equations and Inequalities - 1.5 Applications and Modeling with Quadratic Equations - 1.5 Exercises - Page 135: 60

$70$.

Step 1. Let $R=35,000$, we have $35000=(80-x)(300+20x)$ or $1750=(80-x)(15+x)$ which gives $x^2-65x+1750-1200=0$ or $x^2-65x+550=0$. Step 2. Solve this equation using the quadratic formula to get $x=10\ and\ 55$. Step 3. Since the manager must keep at least 30 units rented, choose $x=10$ which means that the number of units rented is $80-10=70$.