Ответ:

  1. (20 + m) + 34 = m + (20 + 34) = m + 54.
  2. 235 + (565 + x) = (235 + 565) + x = 800 + x.
  3. 412 + k + 158 = (412 + 158) + k = 570 + k.
  4. 1 358 + 2 097 + p = (1 358 + 2 097) + p = 3 455 + p.