I would like to build a calculator using Excel but am unsure of some of the
formulations.

I would like to create a calculator that will return any missing fourth
value when three values are given.

The formula is similar to y = a * b* c

Depending on the real world circumstances, any of three values are present
but the fourth is missing. Is there a formula, expression, function, or
series of if statements that will do this?

Is there a way for the user to type in the values but not destroy the
underlying formulas/functions keeping the caculator working? This might be a
good opportunity for me to learn VBA if this is required. Suggestions would
be helpful.

Thank you,
philr