b2+b3=b2(1+q)=60

b4-b2=b2(q^2-1)=180

(b4-b2)/(b2+b3)=q-1=3

q=4

b2=12

b4=192

b5=768

b2=b1*q; b3=b1*q^2; b4=b1*q^3

 b1*q+ b1*q^2=60;  b1*q^3- b1*q=180

 b1=60/(q+q^2); b1=180/(q^3-q)

 60/(q+q^2)= 180/(q^3-q)

откуда q=4

b1=60/(4+16)=3

b5=b1*q^4=3*256=768