ANSWER KEY to Riddle #11 about the 100 coins:
Split the coins into two separate piles. One pile with 90 coins, and the other pile with 10 coins. Than take the ’10 coin’ pile and flip all of the coins over. After doing that, you will have 2 piles with an equal amount of tales up coins.
This will work 100% of the time.
Try it for yourself!
— The Thinking Hound