Auto Populate Drop Down Fields Based on a Numeric Field

Auto Populate Drop Down Fields Based on a Numeric Field

I am building a sample request form for my company's sales team to use. It contains a numeric field called "2-digit Area #" and a drop down field called "Regional Sales Manager". I would like the drop down box to auto populate with the appropriate RSM once the unique 2-digit Area # is entered. Each RSM covers a range of 10 to 20 areas as their region. These numbers are consecutive for each region (i.e. west region = 1-19, north = 20-39, etc.). I can't for the life of me get this to work. Please point me in the right direction. Thanks in advance. 

Chris