\[\boxed{\text{4.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[999 + 1 = 1\ 000\]
\[900 - 1 = 899\]
\[700 + 80 + 9 = 789\]
\[347 - 7 - 40 = 300\]
\[570 + 30 - 330 =\]
\[= 600 - 330 = 270\]
\[950 + 50 - 660 =\]
\[= 1000 - 660 = 340.\ \ \]