A riddle problem is a question or statement intentionally phrased so as to require ingenuity in ascertaining its answer or meaning, typically presented as a game.  Below is a list of the past problem of the week riddles in descending chronological order.

Riddle: A tree doubled in height each year until it reached its maximum height over the course of ten years. How many years did it take for the tree to reach half its maximum height?
Riddle: When asked how old she was, Suzie replied, "In two years I will be twice as old as I was five years ago." How old is she?
Riddle: Eight years ago, Bill was eight times the age of his son Bill Jr. Today, if you add their ages together, they add up to 52. How old are Bill and his son?
Answer: Bill is 40, and Bill Jr. is 12.
Riddle: 81 x 9 = 801. What must you do to make the this equation true?
Answer: Turn it upside down.  108 = 6 x 18.
Riddle: A girl is twice as old as her brother and half as old as her father. In 50 years, her brother will be half as old as his father. How old is the daughter now?
Riddle: How can you make the following equation true by drawing only one straight line: 5+5+5=550? Can you figure it out?
Answer: Draw a line on the first plus sign that turns it into a 4! The equation then becomes true: 545+5=550. You could also change the equal symbol to a crossed out equal symbol which means "not equal to".
Riddle: How can you take 2 from 5 and leave 4?
Answer: F  I V  E Remove the 2 letters F and E from five and you have IV which is the Roman numeral for four.
Riddle: What is the next number in the series? 7,645 5,764 4,576...
Answer: 6,457. The last digit is moved to the front to make the next number.
Riddle: If seven people meet each other and each shakes hands only once with each of the others, how many handshakes will there have been?
Answer: Twenty one. Most people would think there were 42 handshakes. The first person shakes the hand of 6 others, the second person shakes the hand of 5 remaining people, the third person shakes the hand of 4 remaining people, the fourth person shakes the hand of 3 remaining people, the 5th person shakes the hand of 2 remaining people and the sixth person shakes the hand of 1 remaining person. 6+5+4+3+2+1=21