Unfinished questions

[StephLin]

There are some unfinished questions. Let's use this issue to track them.

• Chapter 3: 3.21
• Chapter 4: 4.08, 4.22
• Chapter 5: 5.05, 5.06, 5.07, 5.08, 5.11, 5.14, 5.15, 5.16, 5.17, 5.18, 5.19, 5.20, 5.21, 5.22
• Chapter 6: 6.07, 6.09, 6.10, 6.11, 6.12, 6.13, 6.14, 6.15, 6.16, 6.17, 6.18, 6.19, 6.20, 6.21, 6.22, 6.23

Please comment below to adopt any of them, and write your solution in your forked repository. Thank you!