Project Euler
This came up in DROD chat a few times - it's essentially a collection of math puzzles about number properties, or in some cases geometry. For example, you might be asked to

find the index of the first Fibonacci number with 1000 digits, or

figure out whether each of a thousand triangles contains the origin or not.

A surprisingly large number of them are solvable just with paper and pencil, but since several of them involve very large numbers, very large data-sets, or both, you're generally encouraged to do some programming to have your computer help you out with searching for the answer.

This combination of mathematical deduction, to reduce the size of your search space, and clever programming to efficiently search said space, or do the computations in a straightforward manner, means that solving any one single puzzle is quite an accomplishment.

Anyway, here's my friend key in case you want to share progresses:

743381_VkXHaIiXuipT7mwLRNssohIBZLm6ZJ8m

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Quick links to my stuff (in case you forgot where it was):

Click here to view the secret text