How to sum up a bunch of currency fields on an entity....

I am wondering how I can go about summing up a bunch of currency fields on an entity and then take that sum and put the total in another currency field.  Example:

If fiedl1, field2, field3, field4, field5 all have a value in their field, then sum those up and place the total in field x.  But, if one of them does not have a value in, then the field x should be an open currency field where the user can enter in a value if need be.

Thank you in advance for your help.