Help writing formula

Hi, everyone!

I know enough about Excel to be dangerous!  I'm comfortable with basic math formulas but this one is special.  What I want to do is take a percent of a cell value.  Easy, right?  No.  The percentage will be based on a selection from a drop-down list of four values (5%, 10%, 15% or 20%, or their decimal equivalent).  The calculation is performed once that percentage is selected.  Next, that value will be subtracted from that original cell value to result in a final amount.  Help!