Помогите 3 и 4 сделать пж

Решите задачу:

$1.\;S=\frac{b_1}{1-q}\\\frac{15}{1-q}=18\\15=18-18q\\18q=3\\q=\frac16\\\\2.\;S=\frac{b_1}{1-q}\\b_6=b_3\cdot q^3\\25\cdot q^3=0,2\\q^3=0,008\\q=0,2\\b_3=b_1\cdot q^2\Rightarrow b_1=\frac{b_3}{q^2}=\frac{25}{0,04}=625\\S=\frac{625}{1-0,2}=\frac{625}{0,8}=781,25\\\\3.\;2,36(13)=2+\frac{3613-36}{9900}=2+\frac{3577}{9900}=2\frac{3577}{9900}$

$4.\;S=\frac{b_1}{1-q}\\S_n=\frac{b_1(1-q^n)}{1-q}\\\begin{cases}\frac{b_1}{1-q}=32\\\frac{b_1(1-q^5)}{1-q}=31\end{cases}\Rightarrow\begin{cases}b_1=32(1-q)\\b_1=\frac{31(1-q)}{1-q^5}\end{cases}\\32(1-q)=\frac{31(1-q)}{1-q^5}\\1-q^5=\frac{31(1-q)}{32(1-q)}\\1-q^5=\frac{31}{32}\\q^5=1-\frac{31}{32}=\frac1{32}\\q=\frac12\\\begin{cases}b_1=16\\q=\frac12\end{cases}$