On Friday I set this puzzle. If you haven’t tried to solve it, have a go now. For everyone else, the answer is after the break.
You and a friend have a pile of 15 pennies and decide to play a little game. You will each take turns removing pennies from the pile. On each turn you or your friend can take 1, 2, or 3 pennies. The loser is the person who takes the last penny. You are allowed to go first. How many pennies should you take?
You want to leave your friend 5 pennies. That way, no matter how many pennies they take, you can leave the final penny to them. Similarly, it is good to leave them with 9 pennies because no matter what your friend does, they must give you the opportunity to leave them with 5. Similarly, leaving your friend with 13 pennies will ensure that you can reach 9, then 5. So, on your first turn, take 2 pennies.
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED
and is available for the Kindle
and USA here
) and on the iBookstore
in the USA here
). You can try 101 of the puzzles for free here