S5=b1(1-q^5)/(1-q)

121/3=b1*(1-(1/3)^5)/(1-1/3)

121/3=b1*(1-1/243)/2/3

121/3=(b1*242/24)/2/3

121/3*2/3=b1*242/243

242/9=b1*242/243

b1=242/9:242/243

b1=242/9*243/242

b1=27

b5=b1*q^4

b5=27*(1/3)^4

b5=27*1/81

b5=1/3