Answer
(a) $f(x)=0.80\cdot [\frac{x}{2}]$
(b) $3.20\ dollars$, $5.60\ dollars$.
Work Step by Step
(a) For $x\in[6,18]$, first we need to find the integer $[\frac{x}{2}]$ then multiply the price per 2 ft. Thus we have $f(x)=0.80\cdot [\frac{x}{2}]$
(b) For $x=8.5$, we have $f(8.5)=0.80\cdot [\frac{8.5}{2}]=0.8\cdot 4=3.20\ dollars$. For $x=15.2$, we have $f(15.2)=0.80\cdot [\frac{15.2}{2}]=0.8\cdot 7=5.60\ dollars$.
