((1+b)/b)x+((1+a)/a)y=b-a, ax(1+b)+by(1+a)=ab(b-a)
x/b-4y/a=5, ax-4by=5ab, ax=5ab+4by; by=a(x-5b)/4.
Substitute for by in the first equation:
ax(1+b)+(a+a2)(x-5b)/4=ab2-a2b,
5ax+4abx-5ab+a2x-5a2b=4ab2-4a2b,
ax(5+4b+a2)=4ab2+a2b+5ab,
x(a2+4b+5)=b(a+4b+5),
x=b(a+4b+5)/(a2+4b+5).
Substitute for ax in the first equation:
(5ab+4by)(1+b)+by(1+a)=ab(b-a),
5ab+4by+5ab2+4b2y=ab2-a2b,
y(4b+4b2)=-4ab2-a2b-5ab,
y=-ab(4b+a+5)/(4b(1+b)),
y=-a(a+4b+5)/(4(1+b)).