Verified answer

Решаем систему:

в6+в7=60

в5+в6=15

b1q^5+b1q^6=60

b1q^4+b1q^5=15

b1q^5(1+q)=60

b1q^4(1+q)=15

1+q=60/(b1q^5)

1+q=15/(b1q^4)

60/(b1q^5)=15/(b1q^4)

4/q=1

q=4

b1*(4^4)(1+4)=15

b1*(4^4)*5=15

b1*(4^4)=3

b1=3/(4^4)

b8=b1q^7=(3/(4^4))*(4^7)=3*(4^3)=3*64=192