Let S = # of student tickets & G = #b of gen'l adm tickets
S + G = 21500
3S + 5G = 78500
Multiply the 1st equation by 5
5S + 5G =107500
3S + 5G = 78500
Subtract the bottom from the top
2S = 29000
Divide by 2
S = 14500
G = 21500 - S = 7000