not drake!
m=number of members
n=number of nonmembers
n+m=25
cost=550=20m+25n
simplify by dividing by 5
110=4m+5n
so we have
n+m=25 and
5n+4m=110
multiply first eequation by -4 and add to second equation
-4n-4m=-100
5n+4m=110+
n+0m=10
n=10
subsitute
n+m=25
10+m=25
m=15
15 members
10 non-members