[SandB0x]

Have a look at the UK's STEP mathematics papers. They're the toughest school-level exams we have, used as entrance tests for a handful of degree courses. There are three papers each year, in increasing level of difficulty and assumed knowledge, and each candidate takes papers I and II or papers II and III.

http://www.admissionstests.cambridgeassessment.org.uk/adt/st...

[technotony]

Thanks for sharing that... I found the paper from 1998 that I took... I don't remember how to answer any of the questions anymore which is pretty scary given I did OK back then!!

[xyzzyz]

Some problems are quite fun, but they spoil them with clues -- it does not feel like solving, more like explaining the solution.