Excel Trick: SumProduct & Double Minus (“–“)

I recently received an email asking if I was aware of a formula that could validate the information in two columns, and return the summed value for all matches. This is easier to grasp in context, which makes the video helpful. I was, admittedly, slightly long-winded in explaining the purpose of the formula. To avoid this lengthy explanation, skip to the two-minute mark once you understand the objective.
The template used in the video is available for download: Download Template
What you will learn in this video:
1. How to use SumProduct as an array formula.
2. What the double hyphen (or double minus) means in Excel formulas.
3. How to create drop-down data validation lists.
4. How to use F9 to audit formulas.