You have 3 pairs of pants or skirts, 4 shirts or blouses, and 5 pairs of shoes. You can use them to wear different outfits.
You are a participant in a peace conference with 10 participants. Everybody shakes everybody else's hand. There are handshakes altogether.
A family of five is taking an extended vacation. Every day at lunch they stand in line at a cafeteria in a different order than ever before. On the last day, however, they can't help repeating a previous order. Their vacation lasted days.