c=number of children
a=number of adults
total number os 30 so
a+s=30
cost for all adult=12a
cost for all child=8c
total cost=12a+8c
280=12a+8c
divide by 4 to make it easier
70=3a+2c
so we have
a+c=30
3a+2c=70
multiply firse equation by -2 and add to second equaiton
-2a-2c=-60
3a+2c=70 +
1a+0c=10
a=10
subsitute
a+c=30
10+c=30
c=20
20 childeren
10 adults