Цепочка: 15, 35, 55, 75, 95.
Вычисления:
15 + 20 = 35
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}15\\ 20\end{array}}\\ \begin{array}{r}35\end{array}\end{array}$
35 + 20 = 55
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}35\\ 20\end{array}}\\ \begin{array}{r}55\end{array}\end{array}$
55 + 20 = 75
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}55\\ 20\end{array}}\\ \begin{array}{r}75\end{array}\end{array}$
75 + 20 = 95
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}75\\ 20\end{array}}\\ \begin{array}{r}95\end{array}\end{array}$