4/7x+1/4y=90

=>3/7x+3/4y=90

=>4/7x+1/4y-3/7x-3/4y=0=1/7x-2/4y=0

=>1/7x=1/2y

=>

4/7x=2y

=>2и 1/4y=90

=>y= 90*4/9=40

=> x=180-40=140

разность = 140-40=100