Verified answer

[tex]\displaystyle b_1=3; b_4=81\\\\b_4=b_1*q^3=3*q^3=81\\\\q^3=27\\\\q=3\\\\S_5=\frac{b_1(q^n-1)}{q-1}=\frac{3*(3^5-1)}{3-1}=\frac{3*242}{2}= 363[/tex]