SUMPRODUCT(...)
How to use SUMPRODUCT function to validate a criteria and sum
Used Functions
Category: Math and trigonometry
Returns the sum of the products of corresponding array components
Syntax
SUMPRODUCT(array1, [array2], [array3], ...)
Practice SUMPRODUCT Formula at Sheetzoom
SUMPRODUCT formula at Microsoft help
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The formula works by validating a criteria for a criteria range and then summing the valid against them for cases it is true. The formula returns an array of 1s and 0s when checking for a criteria and this binary array is multiplied with the values and summed up to give the final answer.