Let x be the minutes spent on call on the cell phone.
The expression for the monthly fee would be:
[tex]0.07x+(30+15)[/tex]This way, if the monthly fee was $74.40, that means
[tex]74.40=0.07x+45[/tex]Solving for x ,
[tex]\begin{gathered} 74.40=0.07x+45 \\ \rightarrow74.40-45=0.07x \\ \rightarrow\frac{29.4}{0.07}=x \\ \Rightarrow x=420 \end{gathered}[/tex]We get that Mr. Perez used his cell phone for 420 minutes
