##### Assuming the first term of each sequence is n=1:
1. $$a_n = 4-4n$$ and $$a_{20} = -76$$
2. $$a_n = \frac23(n-1)$$ and $$a_{20} = \frac{38}{3}$$
3. $$a_n = 36-11n$$ and $$a_{20} = -184$$
4. $$a_n= -5 + \frac72 n$$
5. $$a_n=6 – \frac12 n$$
6. $$a_n = -70+9n$$
7. $$a_n = -5+9n$$
8. $$a_n = 22-4n$$
9. $$a_n = \frac{12}{5} – \frac25 n$$
10. $$2585$$
11. $$n = 5$$
12. $$a_n = 2\cdot 3^n$$
13. $$a_n= \frac{15}{2} \cdot \left( \frac25 \right)^n$$
14. $$a_n = 8 \cdot \left(\frac12\right)^n$$
15. $$a_n = 60 \cdot \left(\frac12\right)^n$$
16. $$a_n = -3072 \cdot \left( -\frac14\right)^n$$
17. $$a_n = \frac45 \cdot 5^n$$
18. $$a_n = \frac32 \cdot 2^n$$ or $$a_n = \frac32 \cdot (-2)^n$$
19. $$a_n = -160 \cdot 2^n$$ or $$a_n = -160 \cdot (-2)^n$$
20. $$a_n = 14\cdot \left(\frac12\right)^n$$ or $$a_n = -14 \cdot \left(-\frac12\right)^n$$
21. $$5115$$ (Note, problem should read “…of the geometric series.”)

There’s also a chance that the teacher uses $$(n-1)$$ more than $$n$$ in the answers to 1-9 and 12-20. If so, then the teacher’s answers can be simplified into these answers.