Ответ:

  1. x + 400 = 500; x = 500 − 400; x = 100.
  2. 600 + x = 900; x = 900 − 600; x = 300.
  3. x − 300 = 400; x = 400 + 300; x = 700.
  4. 800 − x = 200; x = 800 − 200; x = 600.
  5. 700 − x = 200; x = 700 − 200; x = 500.
  6. x − 800 = 0; x = 0 + 800; x = 800.