Ответ:

  1. 840 : x = 900 − 879; 840 : x = 21; x = 840 : 21 = 40.
  2. y − 49 = 1515 : 15; y − 49 = 101; y = 101 + 49 = 150.
  3. 14t − 9t = 1025; 5t = 1025; t = 205.
  4. 3m + m + 6m = 2400; 10m = 2400; m = 240.