b2+b3=105,
(b1+b2)-(b2+b3)=35, b1+b2=140,
b2=b1q, b3=b1q^2,
b1q+b1q^2=105,
b1+b1q=140,
b(1+q)=140, b1=140/(1+q),
b1q(1+q)=105, 140/(1+q) *q(1+q)=105, 140q=105, q=3/4,
b1=140/(1+3/4)=80.
