I spent a couple of hours last Saturday participating in the US Puzzle Championships on line. This is open to anyone, so it's not like I qualified or anything. I did this last year and finished right in the middle of the pack. I probably did the same this time too.
Of the 22 puzzles, I solved about 6. The frustrating part is that I finished these 6 within the first hour, and then spent the next hour and a half spinning my wheels. One deceptively hard puzzle was about coins:
A fictitious country has 3 denominations of coins. You can make 20, 23, and 29 "units" with 3 coins. What are the 3 denominations? I spent tons of time going through different combinations. Use of calculators or computers was against the rules, so I couldn't whip up a quick program to find the answer.
Still, it was fun, and I'll do it again next year.