20.1 problem 636
20.2 problem 637
20.3 problem 638
20.4 problem 639
20.5 problem 640
20.6 problem 641
20.7 problem 642
20.8 problem 643
20.9 problem 644
20.10 problem 645
20.11 problem 646
20.12 problem 647
20.13 problem 648
20.14 problem 653
20.15 problem 654
20.16 problem 655
20.17 problem 656
20.18 problem 657
20.19 problem 658
20.20 problem 659
20.21 problem 660
20.22 problem 661
20.23 problem 662
20.24 problem 663
20.25 problem 664
20.26 problem 665
20.27 problem 666
20.28 problem 667
20.29 problem 668
20.30 problem 669
20.31 problem 670
20.32 problem 671
20.33 problem 672
20.34 problem 673
20.35 problem 674