28.1 problem 798
 28.2 problem 799
 28.3 problem 800
 28.4 problem 801
 28.5 problem 802
 28.6 problem 804
 28.7 problem 805
 28.8 problem 806
 28.9 problem 807
 28.10 problem 808
 28.11 problem 809
 28.12 problem 810
 28.13 problem 811
 28.14 problem 812
 28.15 problem 813
 28.16 problem 814
 28.17 problem 815
 28.18 problem 816
 28.19 problem 817
 28.20 problem 818
 28.21 problem 819
 28.22 problem 820
 28.23 problem 821
 28.24 problem 822