Calculation

I hope I can explain this clearly.  Column A is quantity, Column B is price, and Column C is the total for the quantity*price.  Field A is the total of all entries in Column C.  When testing each field I put a quantity in each box then I removed these quantites.  However, my Total in Field A is not resetting back to zero even though all the amounts in Column A (price) and Column C (total) are zero.  How do I get the Total field to reset (the spot that shows $63.00) to zero if all entries are blank?