In the "Formula" box under Data Validation Custom:

=OR($C$1=1,$C$1=$A$1+1)

HTH
Jason
Atlanta, GA

-----Original Message-----
if cell a1 = b1/2 user should choose if 1st half of
order or 2nd half.
The answer will post as a result in c1. In other words
if a1 = 25 and b1 =
50 the user should choose if c1 equals 1 or 26 (1st or
2nd part of total
units in cell B1)

any help greatly appreciated.

Steven
.