I have a spreadsheet that consists of a series of drop down options. I am using a IF statement

I have a spreadsheet that consists of a series of drop down options. I need to know how to change the values of a certain formula if an option is selected. For example, if a user selects "Not Applicable" the weighted average should change. For instance, if I have 4 questions each value at 25% each, this will change if a user selects "Not Applicable" for one question. The weighted average will then need to be 33.333333% since there are only 3 questions that should be weighted. The same will happen if a user selects "Not Applicable" for 2 questions the weighted average should be 50% for the remaining questions. Below is the formula that I am currently using:

=IF(OR(D39="Yes",D39="Yes - Corrected"),0.25,"")