A more elaborate specification would help, but it looks like...

C2:

=CHOOSE(MATCH(B2,DataValidationList,1),Expression1 ,Expression2,Expression3,Expression4,Expression5)

ExpressionN is an calculation that involves A2 (user entry) which
depends on the choice from B2. DataValidationList is the list used to
data validate B2.

Andrew Hill wrote:
I have set up a validation field giving 5 options from which to choose in
ColB. In ColA a number field requires a manual entry, ColC is the result of
ColA x ColB (which should vary based on the choice made from the drop menu
choice in ColB).

Question is how to apply a differnet (hidden) formula to each of the
different text options in the drop down validation menu?

Many thanks in advance.
Regards ...Andrew