Hi gumby:
I would do it this way:
1. Highlite A20:A60 ... DATA|VALIDATION|Settings|Allow|List
2. Create source list, say starting in cell D24 ... =A26, in cell D25 ... =A33, in cell D26 ... A40, in cell D27 ... =A47, in cell D28 ... =A53
3. and the necessary information related to Input_message, and Error Alert
There may be other ways of doing it -- but this way should do it.
Please post back if it works for you ... otherwise explain a little further ... and let us take it from there!