x-(1/2x-20)-{1/2[x-(1/2x-20)]+20}=50
x-1/2x+20-1/2[x-(1/2x-20)]-20=50
1/2x-1/2[x-(1/2x-20)]=50
x-[x-(1/2x-20)]=100
1/2x-20=100
1/2x=120
x=240