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: 81 x 9 = 801. What must you do to make the this equation true?
Answer: Turn it upside down.  108 = 6 x 18.
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
Riddle: A group of campers have been on vacation so long, that they've forgotten the day of the week. The following conversation ensues. Darryl: What's the day? I don't think it is Thursday, Friday or Saturday. Tracy: Well that doesn't narrow it down much. Yesterday was Sunday. Melissa: Yesterday wasn't Sunday, tomorrow is Sunday. Ben: The day after tomorrow is Saturday. Adrienne: The day before yesterday was Thursday. Susie: Tomorrow is Saturday. David: I know that the day after tomorrow is not Friday. If only one person's statement is true, what day of the week is it?
Answer: It is Wednesday. If it was any other day of the week, more than one statement would be true. To solve the riddle, evaluate each person's statement and write down what day it could be according to the statement. David's statement indicates it could be any day of the week except for Wednesday. When you list the days that it could be according to everyone's statement, it turns out Wednesday is the day mentioned only one time. Darryl: Sunday, Monday, Tuesday, or Wednesday Tracy: Monday Melissa: Saturday Ben: Thursday Adrienne: Saturday Susie: Friday David: Sunday, Monday, Tuesday, Thursday, Friday or Saturday