Условие:

Вычисли устно и объясни решение.
149 + 301 + 203
340 + 129 + 231
199 + 185 + 201
125 + 392 + 75

149 + 301 + 203 = 653
340 + 129 + 231 = 700
199 + 185 + 201 = 585
125 + 392 + 75 = 592
Вычисления:
149 + 301 + 203 = (149 + 301) + 203 = 450 + 203 = 653
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}149\\ 301\end{array}}\\ \begin{array}{r}450\end{array}\end{array}$
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}450\\ 203\end{array}}\\ \begin{array}{r}653\end{array}\end{array}$
340 + 129 + 231 = 340 + (129 + 231) = 340 + 360 = 700
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}129\\ 231\end{array}}\\ \begin{array}{r}360\end{array}\end{array}$
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}340\\ 360\end{array}}\\ \begin{array}{r}700\end{array}\end{array}$
199 + 185 + 201 = (199 + 201) + 185 = 400 + 185 = 585
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}199\\ 201\end{array}}\\ \begin{array}{r}400\end{array}\end{array}$
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}400\\ 185\end{array}}\\ \begin{array}{r}585\end{array}\end{array}$
125 + 392 + 75 = (125 + 75) + 392 = 200 + 392 = 592
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}125\\ 75\end{array}}\\ \begin{array}{r}200\end{array}\end{array}$
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}200\\ 392\end{array}}\\ \begin{array}{r}592\end{array}\end{array}$