Let x and y be the number of tickets for students and for parents respectively.
Therefore, the two equations mentioned in the question are
[tex]\begin{gathered} x+y=100 \\ 2.5x+3y=272.5 \end{gathered}[/tex]
Solve the system of equations as shown below
[tex]\begin{gathered} \Rightarrow x=100-y \\ \Rightarrow2.5(100-y)+3y=272.5 \\ \Rightarrow250+0.5y=272.5 \\ \Rightarrow0.5y=22.5 \\ \Rightarrow y=45 \end{gathered}[/tex]
Finding x,
[tex]\begin{gathered} y=45 \\ \Rightarrow x=100-45=55 \end{gathered}[/tex]
Therefore, there were 55 student tickets sold and 45 parent tickets sold.