Arithmetic sequences

A certain type of bamboo grows $1.3\,\mathrm{m}$ per day during the vegetation period. Concerning the fact that it reached a height of $30\,\mathrm{m}$ after twenty days of regular growth, how tall was it at the beginning of the first day?

The sum of the $2$nd and the $4$th term of an arithmetic sequence is $1.2$, the sum of the first $10$ terms is $3.5$. Find the first term.

Given an arithmetic sequence $\{a_n\}_{n=1}^{\infty}$, where $a_5=\frac{11}2$, $a_{10}=-2$ and $s_k=a_1+\dots+a_k=15$, find $k$.

Find the sum of the $10$th up to $20$th term of an arithmetic sequence given by: $a_{n+1}=a_n+2$ for all $n\in\mathbb{N}$ and $a_1=-4$.