Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.
A non-mathematical one this week. What four-letter word is the same when it is read forwards, backwards and upside down?
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.