Chapter 3 - Polynomial and Rational Functions - 3.6 Variation - 3.6 Exercises - Page 389: 33

$16\ in$

Step 1. Assume $x\ (in)$ is the distance a spring stretches. Step 2. Assume $F\ (lb)$ is the force applied. Step 3. From the given conditions, we have $x=kF$ and $8=k(15)$, thus $k=\frac{8}{15}$ Step 4. For $F=30\ (lb)$ , we have $x=\frac{8}{15}(30)=16\ in$