AHA! Puzzle This
By Michael H. Pryor
MAKE’s favorite puzzles. (When you’re ready to check your answers, visit
makezine.com/06/aha.)
Coins on the Table
Your roommate challenges you to a game with the You sit down at the perfectly round kitchen table
quarters from each of your sock drawers. Whoever and each person takes a turn, placing a quarter down
wins the game gets to keep all the money. (Assume anywhere on the table. No quarters can overlap and
you each have an unlimited supply of quarters.) the entire quarter must rest on the table surface. The
first person that can’t put a quarter down on the table
loses. You each have plenty of quarters and won’t run
out during the game.
Your roommate wants to go first. But you think
about it for a while and realize you would much rather
go first because you’ve figured out a surefire method
to win (without cheating). After some convincing, your
roommate allows you to place the first quarter.
Where do you place it, and what is your
winning strategy?
In-Your-Head Math
How many trailing zeroes are there in 100 factorial ( 100!)? ( 100 factorial is 100 99 98 etc., down to 1.)
For example, 5,030,499,400,000 has five trailing zeroes.
One Mile South
How many points are there on Earth where a
person can walk one mile south, one mile east,
and then one mile north and end up in the same
spot they started in? To be precise, let’s assume
the Earth is a solid smooth sphere, without oceans
or mountains.
The North Pole is one such place where you
can walk one mile south, one mile east, one mile
north and end up where you started. But is there
another starting point that works?
If you think you’ve figured it out, I’ll give you a
hint. There is more than one point. In fact, there
are more than two points.
Calendar Cubes
Each morning I come into work and arrange two normal cubes on my desk to display the current day of the
month. For example, on the 1st of the month, the cubes have a 0 and a 1 on them. On the 16th day of the month,
one cube has a 1 and the other a 6. On the 31st day of the month, a 3 and a 1 appear on the front of the cubes.
Each cube has six sides and one number painted on each side. What are the numbers painted on each cube?
Michael Pryor is the co-founder and president of Fog Creek Software. He runs a technical interview site at techinterview.org.
