I've been a fan of mathematical puzzles for a long time.
Here are four classic puzzles that are among the very best.
A duck is in the center of a circular pond, with a bear
waiting on the shore to kill it. If the duck can reach the shore
without immediately being pounced upon it will fly away to safety,
but it can't get traction to fly from water.
Trying to outwait the bear is not an option: The bear is hungry,
but the duck is hungrier.
The duck paddles at 1 meter per second. If the bear runs at
less than 3.14159 (π) meters per second, the duck will
escape just by paddling away from the bear's starting position.
But the duck can do better than that. What is the fastest-running
bear from whom the duck can escape?
Click here for solution.
The warden has decided to shut down his prison, but hasn't decided
whether to release all 100 prisoners, or to just execute them all.
He decides to stage a contest. He writes the 100 prisoner names
(along with their prisoner ID numbers, 1 to 100)
on 100 slips of paper, shuffles them thoroughly, and places them in 100
numbered drawers.
Each prisoner will be given a chance to open drawers one-by-one
until he either finds his name or has opened fifty drawers.
If all prisoners locate their own slip within fifty drawers then
all hundred of them are set free, but if even one prisoner
fails, then all 100 will be executed.
The prisoners are allowed to plan a strategy, but once the
drawer-opening begins no further communication is permitted.
The prisoners will not be able to observe which drawers their
fellows open; the drawers and their contents will all be restored
to their initial state between prisoners.
For any single prisoner it is easy to see that his chance of success
is 50/100. If the prisoners' chances are independent then their
combined chance will be (50/100)^100, which is less than one chance
in a trillion billion billion. Can they improve on this?
Click here for solution.
The Sultan will show you one thousand of his daughters, one-by-one
in an order determined by random chance,
tell you how many dinars the maiden's dowry is, and ask if you want
to marry her. Your decision is irrevocable: if you say 'Yes' you won't
even meet the other daughters; if you say 'No' you won't be able to pick
that maiden later. In other words, if you reject the first 999, you're
stuck with daughter #1000.
There's one catch. The Sultan wants to know if you're clever enough to
pick the girl with the largest dowry. If you pick any of the other 999
girls, you'll be executed instead of married.
How do you play, and what is your chance of success?
(For a variation, how about if the Sultan lets you live and marry either
of the two highest-dowried daughters?)
Click here for solution.
A spectator chooses five cards from an ordinary deck of fifty-two.
The magician's assistant removes one of the five cards, and arranges the
other four into some order. No other signalling is permitted,
just the choice among 4! = 24
possibilities for ordering the four cards.
The magician comes into the room, inspects the four cards in order,
and announces what the fifth card was. How does he do it.
Can the Duck escape from the Bear?
Can the Hundred Prisoners Survive?
Pick the largest Dowry
Figure the fifth card