Tell if each is an arithmetic sequence. If it is, give the common difference.
5. 1, 2, 4, 8, 16,...
6. 60, 55, 50, 45, 40,....
7. 20, 21.5, 23, 24.5, 26,...
8. 30, 80, 130, 180,...
9. 3, 6, 12, 21, 33, 48,...
10. 5000, 1000, 200, 40,..