2.6.1 problem 60
  2.6.2 problem 61
  2.6.3 problem 62
  2.6.4 problem 63
  2.6.5 problem 64
  2.6.6 problem 65
  2.6.7 problem 66
  2.6.8 problem 67
  2.6.9 problem 68
  2.6.10 problem 69
  2.6.11 problem 70
  2.6.12 problem 71
  2.6.13 problem 72