Practice: Do all of these calculations mod 10 (* means multiply):

1. 8+7 ≡ 5 (mod 10)

2. 3+9 ≡ 2 (mod 10)

3. 6+7 ≡ 3 (mod 10)

4. 3*6 ≡ 8 (mod 10)

5. 4*8 ≡ 2 (mod 10)

6. 7*9 ≡ 3 (mod 10)

7. 2 - 8 ≡ 4 (mod 10)

8. 4 - 6 ≡ 8 (mod 10)

9. 5 - 8 ≡ 7 (mod 10)

10. 7 + (3x8) + 9 ≡ 0 (mod 10)
(7+24+9 ≡40 ≡0)

11. 4*3 + 6 + 7*3 ≡ 9 (mod 10)
(12+6+21 ≡2+6+1 ≡9)

12. 4 - (3*9) ≡ 7 (mod 10)
(4-27 ≡4-7 ≡0-3 ≡10-3 ≡7)

13. 2 - (7*4) ≡ 4 (mod 10)
(2-28 ≡2-8 ≡0-6 ≡10-6 ≡4)

14. 0 - (4*8) ≡ 8 (mod 10)
(0-32 ≡0-2 ≡10-2 ≡8)

Find the missing number N in mod 10 arithmetic. If there is more than one answer, find all of the answers:

15. 3*N ≡8 (mod 10) ... think 3*6 = 18 ... N ≡6

16. 3*N ≡4 (mod 10) ... think 3*8 = 24 ... N ≡8

17. 3*N ≡ (mod 10)1 ... think 3*7 = 21 ... N ≡7

18. 7*N ≡3 (mod 10) ... think 7*9 = 63 ... N ≡9

19. 7*N ≡8 (mod 10) ... think 7*4 = 28 ... N ≡4

20. 2*N ≡6 (mod 10) ... think 2*3 = 6 and 2*8 = 16 ... N ≡3 and 8

Do all of these calculations mod 7:

21. 6+4 ≡ 3 (mod 7)

22. 5+3 ≡ 1 (mod 7)

23. 4*6 ≡ 3 (mod 7)

24. 5*5 ≡ 4 (mod 7)

25. 3-6 ≡ 4 (mod 7)

26. 1-5 ≡ 3 (mod 7)

Do all of these calculations mod 8:

27. 7+4 ≡ 3 (mod 8)

28. 6*5 ≡ 6 (mod 8)

29. 4*7 ≡ 4 (mod 8)

30. 2-5 ≡ 5 (mod 8)