John Allen Paulos in his excellent Who's Counting column at ABC News:
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?
More here.