Chapter 1 - Equations and Inequalities - 1.5 Applications and Modeling with Quadratic Equations - 1.5 Exercises - Page 133: 51

(a) $19.2\ hr$ (b) $84.33\ ppm$

(a) Given $T=0.00787x^2-1.528x+75.89$, for $x=50\ ppm$, we have $T=0.00787(50)^2-1.528(50)+75.89\approx19.2\ hr$ (b) let $T=3$, we have $3=0.00787x^2-1.528x+75.89$ or $0.00787x^2-1.528x+72.89=0$ which can be solved using the quadratic formula to get $x\approx84.33\ ppm$ (discard the other answer which is not in the range of $[50,100]$)