There are 20 people in an empty, square room. Each person has full sight of the entire room and everyone in it without turning his head or body, or moving in any way (other than the eyes). Where can you place an apple so that all but one person can see it?
What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?