Take my macro:
http://www.sulprobil.com/html/redw.html

Write in cell A1:
=INDEX({"A","B","C","D"},INT(redw(10%,20%,30%,40%) *4)+1)

Copy A1 down to A20, for example. On average (!) you will
receive in this example 10% grade A, 20% grade B, etc.

If you want numbers to change each time you hit F9, place
the command Application.Volatile at beginning of that
macro.

HTH,
sulprobil