The condo I stayed in had a keypad in which we were given a unique code. While talking to one of my friends, she became worried about the code being easy to figure out. No fear, I told her, math can put your worry to rest. The code was 6 digits. So using the multiplication counting principle, there are 10 * 10 * 10 * 10 * 10 * 10 = 10 ^ 6 = 1,000,000 different codes. The odds of someone randomly punching in out code was 1/1,000,000 or simple said, one in a million. The probability showed that we were safe. Even if someone had the first three digits of the code that still left a 1/(10 * 10 * 10) or 1 in 10,000 chance of getting the last three digits correctly.
This led to my friend rolling her eyes at me. There is also a fabulous book entitled "One in a Million" that I highly recommend.
I love probability.