The Filipiak Number Exchange is a simple yet challenging puzzle. Unscramble the randomly placed numbers 1 through 24 and get them in order in the fewest number of exchanges. The app keeps count of the exchanges and shows the elapsed game time. A new pattern is generated for every play. Based on a 1942 puzzle book written by Anthony S. Filipiak. When a number is placed in its correct location on a 4 by 6 board, the color of the number changes from black to blue. Any pattern can be unscrambled in 23 exchanges. But 20 and 21 are often possible--I've seen 19 once. Try to devise a strategy to do better than 20. -- Sal Paradise