Q=90-P
F(會費)=CS=(90-P)*(90-P)/2
π=P*(90-P)+(90-P)*(90-P)/2-30*(90-P)=30P-0.5P2+1350
dπ/dP=0, 30-P=0, P=30, Q=60
F=60*60/2=1800
DWL=0