Respuesta :
Let's begin by listing out the information given to us:
[tex]\begin{gathered} c(x)=0.05x+49.95 \\ \text{when x = 300 minutes, we simply substitute x for 300, we have:} \\ c(300)=0.05(300)+49.95 \\ c(300)=15+49.95=64.95 \\ \text{When Demi spent 300 minutes on the phone, her bill is \$64.95} \end{gathered}[/tex]For the second part, we have:
[tex]\begin{gathered} c(x)=0.05x+49.95 \\ \text{when bill is \$78.05, we simply substitute c(x) for \$78.05, we have:} \\ 78.05=0.05x+49.95 \\ 0.05x=78.05-49.95 \\ 0.05x=28.1 \\ x=\frac{28.1}{0.05}=562 \\ \text{That tells us that Demi spent 562 minutes on the phone} \end{gathered}[/tex]