Answer :
First, we must find the total amount C charged in a month for m minutes that Ahmad spend in the phone.
We know that the monthly fee is $23 and there is an additional charge of $0.05 for every minute of use. Therefore, we have:
[tex]C=0.05m+23[/tex]Since the least he has been charged in a month is $96.75, we have:
[tex]\begin{gathered} C\ge96.75 \\ 0.05m+23\ge96.75 \\ 0.05m\ge96.75-23 \\ 0.05m\ge73.75 \\ m\ge\frac{73.75}{0.05} \\ m\ge1475\text{ minutes} \end{gathered}[/tex]