Chapter 1 - Equations and Inequalities - 1.5 Applications and Modeling with Quadratic Equations - 1.5 Exercises - Page 134: 52

(a) $10.3\ hr$ (b) $722.2\ ppm$

(a) Given $T=0.0002x^2-0.316x+127.9$, for $x=600\ ppm$, we have $T=0.0002(600)^2-0.316(600)+127.9=10.3\ hr$ (b) let $T=4$, we have $4=0.0002x^2-0.316x+127.9$ or $0.0002x^2-0.316x+123.9=0$ which can be solved using the quadratic formula to get $x\approx722.2\ ppm$ (discard the other answer which is not in the range of $[500,800]$)