Solution on Monday.
Imagine a test with 26 questions. When you get a question wrong you are deducted 5 points. When you get a question right you are given 8 points. If you answered all of the questions, and had a final score of 0 points, how many questions did you answer correctly?
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.