B1+b2+b3+b4 = 7680
b5+b6+b7+b8 = 480
-------------------------------
b9+b10+b11+b12 --?
b1*(1+q+q^2+q^3) = 7680
b1*q^4*(1+q+q^2+q^3) = 480
q^4 = 480/7680 = 6/96 = 1/16 (q = 1/2)
b9+b10+b11+b12 = b1*q^8*(1+q+q^2+q^3) = b1*q^4*(1+q+q^2+q^3) * q^4 =
480*1/16 = 480/16 = 30