2.1.1 problem 3
  2.1.2 problem 4
  2.1.3 problem 5
  2.1.4 problem 6
  2.1.5 problem 7
  2.1.6 problem 8
  2.1.7 problem 9
  2.1.8 problem 10
  2.1.9 problem 11
  2.1.10 problem 12
  2.1.11 problem 13
  2.1.12 problem 14
  2.1.13 problem 15
  2.1.14 problem 16
  2.1.15 problem 17
  2.1.16 problem 18
  2.1.17 problem 19
  2.1.18 problem 20
  2.1.19 problem 21
  2.1.20 problem 27
  2.1.21 problem 28
  2.1.22 problem 29