I think this assignment is so tough because it looks like a string concatenation problem and most students don't make the leap to realize that it's actually a place-value problem. I've had students write all the examples and still not get the problem. My first hint is, "Remember when you wrote expanded notation in elementary school?" This works for about 15% of them. I then mention place-value and, if that doesn't work, I ask them which place each digit in the function call ends up in. Usually, at some point, they have a light-bulb moment and are kind of embarrassed. This is one that some students never "get" until you tell them, but all of them understand after they see it. Todd <div class="gmail_quote">On Jan 8, 2008 10:23 PM, John Clements <<a href="mailto:clements@brinckerhoff.org">clements@brinckerhoff.org</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <div class="Ih2E3d"> On Jan 8, 2008, at 7:08 PM, William Stanley wrote: > Exercise 2.2.4.   Define the program convert3. It consumes three > digits, starting with the least significant digit, followed by the > next most significant one, and so on. The program produces the > corresponding number. For example, the expected value of > (convert3 1 2 3) > is 321. Use an algebra book to find out how such a conversion works. > > > Okay I had gone past this exercise but as I couldn't figure it out > it was bugging me... and I must say its been a while since I > regularly used algebra but after racking my brain for a while and > googling for even longer I throw myself at your mercy... I am sure > it is rather simple but I am stumped... sometimes I wish I could > peek at those answers... even just to see if I got it right. > > > Travis Stanley </div>I hate to be the design-recipe-bot, but... have you tried following the design recipe?  What are the examples you came up with?  Are there simple examples that you know how to do? John Clements _________________________________________________  For list-related administrative tasks:  <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank"> http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </blockquote></div>