[plt-scheme] Combining digits to form numbers

I was surprised to see all of the answers posted here.

This is one of the best problems in HtDP because it draws out whether
or not you are really following the recipe; because if you are you
will produce the correct solution.

One of the reasons for this is that the "intuitive" approach would be
to write the procedure so that the first digit is least significant;
when you don't write it this way the solution becomes less obvious. It
was really helpful talking through this with folks.