"When David was eight years old, he hammered a nail in a tree to mark his height. Seven years later at age fifteen he returned to see how mush higher the nail would be. If the tree grew at nine centimetres each year since he was eight, how high would the marking be?"
You are having a bonfire and invite twenty of your classmates. They are all quite thirsty. The first asks for 1/2 cup of water, the second asks for 1/4 cup of water, the third for 1/8 cups of water ... etc. How manu cups of water do you need in total?