To enter the house of the professor, you first have to walk up a stairs which consists out of twenty steps. The professor thinks his stairs are boring so he decides to paint them. He buys two colors, yellow and green. Every step will be either painted yellow or green and moreover the professor does not want to have two yellow steps directly after each other. In how many ways can the professor paint his stairs? And what is the name of the professor?

If you would fold a letter in two fifty times, how thick should the enveloppe be that can fit the letter? Assume that the thickness of a piece of paper is about 0.05 mm.

More than twenty years ago, Tim asked his grandfather who was celebrating his birthday that day: "How old are you now?" Grandfather smiled and said: "Add the digits of my year of birth and multiply the answer by 4." Tim would have looked so uncomfortable that his Grandfather immediately added, "By the way, this trick also works with the year we have on the calendar now." What was grandfathers age?