Olympiad level counting: How many subsets of {1,…,2000} have a sum divisible by 5?

Timestamps
0:00 - Puzzle statement and motivation
4:31 - Simpler example
6:51 - The generating function
11:52 - Evaluation tricks
17:24 - Roots of unity
26:31 - Recap and final trick
30:13 - Takeaways