∵2x+y/2=5x+4y/4=1
∴2x+y=2
5x+4y=4
∴x=4/3
y=-2/3
∴x+2y+1/2x-3y+7
=(4/3-4/3+1)/(8/3+2+7)
=1/(35/3)
=3/35