At some point during a baseball season, a player has a batting average of less than 80%.  Later during the season, his average exceeds 80%.  Prove that at some point, his batting average was exactly 80%.

Also, for which numbers other than 80% does this property hold?

A game is played as follows. N people are sitting around a table, each with one penny.  One person begins the game, takes one of his pennies (at this time, he happens to have exactly one penny) and passes it to the person to his left.  That second person then takes two pennies and passes them to the next person on the left.  The third person passes one penny, the fourth passes two, and so on, alternating passing one and two pennies to the next person. Whenever a person runs out of pennies, he is out of the game and has to leave the table. The game then continues with the remaining people.

A game is terminating if and only if it ends with just one person sitting at the table (holding all N pennies).  Show that there exists an infinite set of numbers for which the game is terminating.

Let’s say that a number is squarish if it is the product of two consecutive numbers.  For example, 6 is squarish, because it is 2*3.

A friend of mine at Microsoft recently had a birthday.  He said his age is now squarish.  Moreover, since the previous time his age was a squarish number, a squarish number of years has passed.  How many years would he have to wait until his age would have this property again?

A building has 16 rooms, arranged in a 4×4 grid.  There is a door between every pair of adjacent rooms (“adjacent” meaning north, south, west, and east, but no diagonals).  Only the room in the northeast corner has a door that leads out of the building.

In the initial configuration, there is one person in each room.  The person in the southwest corner is a psycho killer.  The psycho killer has the following traits:  If he enters a room where there is another person, he immediately kills that person .  But he also cannot stand the site of blood, so he will not enter any room where there is a dead person.

As it happened, from that initial configuration, the psycho killer managed to get out of the building after killing all the other 15 people.  What path did he take?

You have two jars.  One contains vinegar, the other oil.  The two jars contain the same amount of their respective fluid.

Take a spoonful of the vinegar and transfer it to the jar of oil.  Then, take a spoonful of liquid from the oil jar and transfer it to the vinegar jar.  Stir.  Now, how does the dilution of vinegar in the vinegar jar compare to the dilution of oil in the oil jar?

You’re given a procedure that with a uniform probability distribution outputs random numbers between 0 and 1 (to some sufficiently high degree of precision, with which we need not concern ourselves in this puzzle).  Using a bounded number of calls to this procedure, construct a procedure that with a uniform probability distribution outputs a random point within the unit circle.

Initially, you’re somewhere on the surface of the Earth.  You travel one kilometer South, then one kilometer East, then one kilometer North.  You then find yourself back at the initial position.  Describe all initial locations from which this is possible.

The games played in the soccer world championship form a binary tree, where only the winner of each game moves up the tree (ignoring the initial games, where the teams are placed into groups of 4, 2 of which of which go onto play in the tree of games I just described).  Assuming that the teams can be totally ordered in terms of how good they are, the winner of the championship will indeed be the best of all of the teams.  However, the second best team does not necessarily get a second place in the championship.  How many additional games need to be played in order to determine the second best team?

You’re given a 3x3x3 cube of cheese and a knife.  How many straight cuts with the knife do you need in order to divide the cheese up into 27 1x1x1 cubes?

You have 12 coins, 11 of which are the same weight and one counterfeit coin which has a different weight from the others.  You have a balance that in each weighing tells you whether the two sides are of equal weight, or which side weighs more.  How many weighings do you need to determine:  which is the counterfeit coin, and whether it weighs more or less than the other coins.  How?

A room has 100 light switches, numbered by the positive integers 1 through 100.  There are also 100 children, numbered by the positive integers 1 through 100.  Initially, the switches are all off.  Each child k enters the room and changes the position of every light switch n such that n is a multiple of k.  That is, child 1 changes all the switches, child 2 changes switches 2, 4, 6, 8, …, child 3 changes switches 3, 6, 9, 12, …, etc., and child 100 changes only light switch 100.  When all the children have gone through the room, how many of the light switches are on?

Each of two players picks a different sequence of two coin tosses.  That is, each player gets to pick among HH, HT, TH, and TT.  Then, a coin is flipped repeatedly and the first player to see his sequence appear wins.  For example, if one player picks HH, the other picks TT, and the coin produces a sequence that starts H, T, H, T, T, then the player who picked TT wins.  The coin is biased, with H having a 2/3 probability and T having a 1/3 probability.  If you played this game, would you want to pick your sequence first or second?

100 coins are to be distributed among some number of persons, referred to by the labels A, B, C, D, ….  The distribution works as follows.  The person with the alphabetically highest label (for example, among 5 people, E) is called the chief.  The chief gets to propose a distribution of the coins among the persons (for example, chief E may propose that everyone get 20 coins, or he may propose that he get 100 coins and the others get 0 coins).  Everyone (including the chief) gets to vote yes/no on the proposed distribution.  If the majority vote is yes, then that’s the final distribution.  If there’s a tie (which there could be if the number of persons is even), then the chief gets to break the tie.  If the majority vote is no, then the chief gets 0 coins and has to leave the game, the person with the alphabetically next-highest name becomes the new chief, and the process to distribute the 100 coins is repeated among the persons that remain.  Suppose there are 5 persons and that every person wants to maximize the number of coins that are distributed to them.  Then, what distribution should chief E propose?

N people team up and decide on a strategy for playing this game.  Then they walk into a room.  On entry to the room, each person is given a hat on which one of the first N natural numbers is written.  There may be duplicate hat numbers.  For example, for N=3, the 3 team members may get hats labeled 2, 0, 2.  Each person can see the numbers written on the others’ hats, but does not know the number written on his own hat.  Every person then simultaneously guesses the number of his own hat.  What strategy can the team follow to make sure that at least one person on the team guesses his hat number correctly?

Consider a game that you play against an opponent.  In front of you are an even number of coins of possibly different denominations.  The coins are arranged in a line.  You and your opponent take turns selecting coins.  Each player takes one coin per turn and must take it from an end of the line, that is, the current leftmost coin or the current rightmost coin.  When all coins have been removed, add the value of the coins collected by each player.  It is possible that you and your opponent end up with the same value (for example, if all coins have the same denomination).  Develop a strategy where you take the first turn and where your final value is at least that of your opponent (that is, don’t let your opponent end up with coins worth more than your coins).