A master puzzler once showed his apprentice the following 3 numbers:
1 5 9
and asked him "Do you know what the next one is?"
The apprentice replied, "With only three numbers it would be very difficult to figure out the sequence but it appears to be an arithmetic series with each next number being 4 more than the previous."
The master puzzler smiled knowingly and said "Some say the fourth number of the sequence is 3 times the second number, others claim it is the sum of the first three. Do you know the fifth, which is also the last number?"
The apprentice wrote four numbers on a piece of paper and was totally baffled as to what the last number could be. "Why would you think I should know this?" he asked.