We have the following data:
There is a base charge of $6. This is equivalent to 600 cents
Also, for messages over 700, the charge is 5 cents.
We are told that she owes $8.15. This means she owes 815 cents.
Thus, we can set up the equation
[tex]\begin{gathered} 815=600+5x \\ \text{where x is the number of messages over 700} \end{gathered}[/tex]If we solve for x
[tex]\begin{gathered} 5x=815-600 \\ 5x=215 \\ x=\frac{215}{5} \\ x=43 \end{gathered}[/tex]The total number of messages for the month of March will be
[tex]700+x[/tex]=
[tex]700+43[/tex]=> 743 messages were sent in the month of March.
