how do i allow fields to be either input or output in excel?
 

i am trying to build a series of financial calculators in excel. Each has
3-6 fields and 1 output field. What i want to do is allow users the
flexibility to leave whichever field blank that they wish and the model will
solve for that blank field. so the same model can be used a number of
different ways and to solve for any of the possible variables. i see this
feature in online calculators but i don't know how to set up my models this
way. Any help would be greatly appreciated.
Thank u!