2.1 problem 1
  2.2 problem 2
  2.3 problem 3
  2.4 problem 4
  2.5 problem 5
  2.6 problem 6
  2.7 problem 8
  2.8 problem 9
  2.9 problem 10
  2.10 problem 11
  2.11 problem 13
  2.12 problem 14
  2.13 problem 15
  2.14 problem 16
  2.15 problem 18
  2.16 problem 19
  2.17 problem 20
  2.18 problem 21
  2.19 problem 22
  2.20 problem 23
  2.21 problem 24
  2.22 problem 25
  2.23 problem 26
  2.24 problem 27