A man works on the 10th floor and always takes the elevator down to ground level at the end of the day. Yet every morning, he only takes the elevator to the 7th floor and then walks up the stairs to the 10th floor, even when in a hurry. But he will go all the way to the 10th floor when others are in the elevator with him or on a rainy day. Why?
You have a basket containing ten apples. You have ten friends, who each desire an apple. You give each of your friends one apple.
Now all your friends have one apple each, yet there is an apple remaining in the basket.
You have 10 bags full of coins, in each bag are 1,000 coins. But one bag is full of forgeries, and you can’t remember which one. But you do know that a genuine coins weigh 1 gram, but forgeries weigh 1.1 grams. To hide the fact that you can’t remember which bag contains forgeries, you plan to go just once to the central weighing machine to get ONE ACCURATE weight. How can you identify the bag with the forgeries with just one weighing? And what if you didn’t know how many bags contain forgeries?