Using blank/not blank in a numeric equation

Has anyone created an equation that adds currency fields if date fields are not blank? According to what I read, blank/not blank can’t be used in an equation?
I have three payment fields and each has a corresponding date field that identifies the date the payment was received. I need to calculate total paid to date by adding the payment fields that have a corresponding date. Anyone have a sample of how you’ve handled this?

May be worth checking this out: