The total fee has two parts:
0. A monthly fee of $18, that does not depends on the minutes of use.
,1. A variable fee of 0.07*m, where m are the minutes of use.
Then, we can express the fee C(m) as:
[tex]C(m)=18+0.07m[/tex]The least she has been charged is $121.25.
Then, we can express this as C(m) ≥ 121.25.
We can find the number of minutes m as:
[tex]\begin{gathered} C(m)\ge121.25 \\ 18+0.07m\ge121.25 \\ 0.07m\ge121.25-18 \\ 0.07m\ge103.25 \\ m\ge\frac{103.25}{0.07} \\ m\ge1475 \end{gathered}[/tex]Answer: she has used the phone at least for 1475 minutes.
