b1+b2=90,
b2+b3=72;
b2=b1q,
b3=b1q^2;
b1+b1q=90,
b1q+b1q^2=72;
b1(1+q)=90,
b1q(1+q)=72;
b1=90/(1+q),
b1=72/(q(1+q));
90/(1+q)=72/(q(1+q)),
90q=72,
q=4/5;
b1=90/(1+4/5)=50,
b2=50*4/5=40,
b3=40*4/5=32.
