Adding values when particular Drop Down items are selected

Hey There, I am in the process of getting a wedding budget together. I am stuck on a formula to create.

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I need the columns to add up depending on whether "Colb & Court" is selected or "Mom & Dad". We are comparing two different venues (column B & C) and dividing up costs on each (between Colb & Court and Mom & Dad). I need them to automatically add up in B7 and C7, and B8 and C8 depending on which payer was chosen. How would I create the formula to handle this?