x - osoby, które kupiły bilety normalne
y - osoby, które kupiły bilety ulgowe
8 - bilet normalny
5 - bilet ulgowy
\left \{ {{x+y=420} \atop {8x+5y=2640}} \right.
\left \{ {{x=420-y} \atop {8(420-y)+5y=2640}} \right.
\left \{ {{x=420-y} \atop {3360-8y+5y=2640}} \right.
\left \{ {{x=420-y} \atop {3360-3y=2640-3360}} \right.
\left \{ {{x=420-y} \atop {-3y=-720}} \right.
\left \{ {{x=420-y} \atop {y=240}} \right.
\left \{ {{x=420-240} \atop {y=240}} \right.
\left \{ {{x=180} \atop {y=240}} \right.