1. Watch this video to learn how to prove sets equinumerous by establishing a bijection:

Also, read 14.1 Sets with Equal Cardinalities.

2. Watch this video:

3. Using what you know about the rational numbers, try to figure out why the Cartesian product of countably many sets must be countable. After you've thought about it, check out this Web page for an explanation.

4. Watch this video:

Or this one:

5. Watch this video:

6. Watch this video: