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.
Yesterday my friend and I started to walk down a road. We began our walk at the same time, from the same point, and headed in the same direction. I walk at 5 km/h and my friend walks at 6 km/h. Throughout the walk a dog ran between the two of us again and again, with a constant speed of 10 km/h. How far did the dog travel in 1 hour?
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.