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Daily mathematics
Four problems: one below the level you are working at, two at it in different fields, and one above. The last one is meant to defeat you more often than not — that is what it is for.
The set is a deterministic function of the date and what you have already solved, so reloading does not reroll it and the same day gives the same problems on any device. It is computed in your browser; nothing about your progress leaves it.