Ответ:

  1. 13 - 6 + 5 = 7 + 5 = 12
  2. 12 - 9 + 8 = 3 + 8 = 11
  3. 11 - 7 + 9 = 4 + 9 = 13
  4. 7 + 7 - 8 = 14 - 8 = 6
  5. 80 - 20 + 10 = 60 + 10 = 70
  6. 90 - 30 - 40 = 60 - 40 = 20