We basically have to solve the given system of equations and essentially find the value of s.
[tex]\begin{gathered} s+t=315 \\ 5s+8t=2211 \end{gathered}[/tex]The solution is
[tex]\begin{gathered} s+t=315\rightarrow t=315-s \\ \therefore\rightarrow \\ 5\cdot s+8(315-s)=2211\rightarrow5\cdot s+2520-8s=221\rightarrow3s=2520-221=2299 \\ s=\frac{2299}{3}\approx766.34 \end{gathered}[/tex]Therefore, student tickets sold were about 767!