Put labels in column A, and use column B for entry - let's say B2:B5 with y in B2, etc.

Then in C2 to C5, use the formulas

=IF(B2<"",B2,IF(COUNTA($B$2:$B$5)=3,B3*B4*B5,"Ent ries not valid"))
=IF(B3<"",B3,IF(COUNTA($B$2:$B$5)=3,B2/B5/B6,"Entries not valid"))
=IF(B4<"",B4,IF(COUNTA($B$2:$B$5)=3,B2/B3/B5,"Entries not valid"))
=IF(B5<"",B5,IF(COUNTA($B$2:$B$5)=3,B2/B3/B4,"Entries not valid"))

HTH,
Bernie
MS Excel MVP


"philr" wrote in message
...
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