Answer
a. $f(x)=100+0.8x$
b. $g(x)=40+0.9x$
c. $600$ dollars, $580$ dollars.
Work Step by Step
a. For plan A, the annual price is $f(x)=100+0.8x$ where $x$ is the dollars of merchandise purchased.
b. For plan B, the annual price is $g(x)=40+0.9x$ where $x$ is the dollars of merchandise purchased.
c. Let the above cost to be equal, we have $100+0.8x=40+0.9x$ which gives $0.1x=60$ and $x=600$ dollars. The total would be $f(600)=100+0.8(600)=580$ dollars.
