Five or Six Reasons Why Parity Puzzles Are Fun

May 2, 2010 — -- In belated recognition of April being Math Awareness Month, my column this May will deal with parity.

The notion refers to the evenness or oddness of a number, say April the fourth month versus May the fifth. Despite its simplicity parity plays an important role in many areas of mathematics.

It also lends itself to some nice little puzzles, including Rubik's cube and the 15 puzzle. Here are five or six easy examples. The sixth one is fuzzy and involves politics and the Supreme Court, so it doesn't really count.

The answers to the puzzles appear at the end of the column, but don't peek first ... Unless, of course, you feel like peeking.

1. A loose leaf notebook consists of 100 sheets of paper. Number them, front and back, from 1 to 200. Tear out any 25 of the sheets, and add up the 50 page numbers on them. Can you choose the sheets so that the sum of the 50 numbers is 2010?

2. Consider the sum of the first 10 numbers: 1+2+3+4+5+6+7+8+9+10. Can you change some of the plus signs to minus signs so that the resulting sum is 0? For example, 1+2-3-4-5-6+7- 8+9+10=3. This is close, but not 0.

3. Before you is a regular 8-by-8 checkerboard with two diagonally opposite squares missing. Also before you are 31 dominoes, each 2 squares long, 1 square wide.

Since each domino covers two squares when placed on the checkerboard, 31 dominoes should be enough to cover this mutilated checkerboard. (A regular board has 8x8=64 squares, so this one has 64-2=62 squares, and 2x31=62.) So, can you cover this mutilated checkerboard with the 31 dominoes?

4. Two equally matched teams, A and B, play in a best-of-seven World Series. The probability that team A (or team B) wins any given game is 50%, and the first team to win four games wins the Series. The series ends at that point. Is it more likely that the series will end in six games or seven?

5. A dozen prisoners are told that they'll be lined up in the morning, each facing the backs of those ahead of them in line. They're also told that either a red hat or a blue hat will be placed on each of them. They won't be able to see the color of their own hat, but will be able to see the color of the hats of those in front of them.

After lining up the prisoners in this way, the guards will go to the last person in line and ask him what color his hat is. If he gives the correct color, he will be released, but if he answers incorrectly, he will be killed. Then the next to last person will be asked the color of his hat and released if he answers correctly and killed if not, and so on up the line.

The prisoners are told of this impending procedure the night before and try to decide on a strategy that will save as many of their lives as possible. What should they decide?

6. The Supreme Court has nine justices. The majority rules, and on many issues, the court splits 5-4, with the five more conservative justices prevailing. President Obama will decide soon on whom he will nominate to replace Justice Stevens, one of the four more liberal justices. What is his best shot at changing the court and its rulings in the short term.