Re-arranging and grouping the numbers in pairs such that each group
contains a pair of rational numbers with equal denominators, we have
$$\displaystyle\frac{-3}{10}+\displaystyle\frac{7}{15}+\displaystyle\frac{3}{-20}+\displaystyle\frac{-9}{10}+\displaystyle\frac{13}{15}+\displaystyle\frac{13}{-20}$$
$$=\begin{pmatrix}\displaystyle\frac{-3}{10}+\displaystyle\frac{-9}{10}\end{pmatrix}+\begin{pmatrix}\displaystyle\frac{7}{15}+\displaystyle\frac{13}{15}\end{pmatrix}+\begin{pmatrix}\displaystyle\frac{3}{-20}+\displaystyle\frac{13}{-20}\end{pmatrix}$$
$$=\begin{pmatrix}\displaystyle\frac{-3}{10}+\displaystyle\frac{-9}{10}\end{pmatrix}+\begin{pmatrix}\displaystyle\frac{7}{15}+\displaystyle\frac{13}{15}\end{pmatrix}+\begin{pmatrix}\displaystyle\frac{-3}{20}+\displaystyle\frac{-13}{20}\end{pmatrix}$$
$$=\displaystyle\frac{(-3)+(-9)}{10}+\displaystyle\frac{7+13}{15}+\displaystyle\frac{(-3)+(-13)}{20}$$
$$=\displaystyle\frac{-12}{10}+\displaystyle\frac{20}{15}+\displaystyle\frac{-16}{20}$$
$$=\displaystyle\frac{-6}{5}+\displaystyle\frac{4}{3}+\displaystyle\frac{-4}{5}$$ [Expressing each rational in lowest terms]
$$=\begin{pmatrix}\displaystyle\frac{-6}{5}+\displaystyle\frac{-4}{5}\end{pmatrix}+\displaystyle\frac{4}{3}$$
$$=\displaystyle\frac{(-6)+(-4)}{5}+\displaystyle\frac{4}{3}=\displaystyle\frac{-10}{5}+\displaystyle\frac{4}{3}$$
$$=-2+\displaystyle\frac{4}{3}=\displaystyle\frac{-6+4}{3}=\displaystyle\frac{-2}{3}$$