a=number of adult tickets
s=number of student tickets
number of adult tickets was 120 less than twice student
a=-120+2s
total was 840
840=a+s
subsitute -120+2s for a
840=-120+2s+s
840=-120+3s
add 120 to both sides
960=3s
divide both sides by 3
320=s
sub back
a=-120+2s
a=-120+2(320)
a=-120+640
a=520
520 adult tickets
320 student tickets
