Question 271125
Final members are X
Initially, members are X + 5
Initial rent i/s 600/(X + 5)
Final rent is 600/X
so 600/X - 600/(X + 5) = 4
600(X + 5) - 600X = 4X(X+5)
4X^2 +5X - 3000 = 0 
(X + 30)(X - 25) = 0
X = -30, 25
So X = 25.
check, with 30 members, cost would be $20
with 25 members cost would be $24