Consider a tower of height n on the left hand side of a square, and we wish to create a horizontal line along the bottom of the square. We can push any block at the top of a column 1 space to the right, and then gravity takes over. How many ways can we do this? So far I have for n=1, 0,1,2,5, and I guess n=5 is 15 (Catalan numbers? A000108) If we label the blocks for convience, then the 5 options for n=4 are: 444332 444322 443222 433222 433322 Jon Perry perry@globalnet.co.uk http://www.users.globalnet.co.uk/~perry/maths/ http://www.users.globalnet.co.uk/~perry/DIVMenu/ BrainBench MVP for HTML and JavaScript http://www.brainbench.com