x- lei costa un creion, y- lei costa o carte, z- lei costa un stilou
x+y=39500 y=39500-x y=39500-x
y+z=65500 => y=65500-z => y=65500-30800+x =>
z+x=30800 z+x=30800 z=30800-x
y=39500-x 34700+x=39500-x 2x=4800
y=34700+x => y=34700+x => y=34700+x =>
z=30800-39500+y z=y-8700 z=y-8700
x=2400
y=37100
z=28400