SUMIF(...)
Calculate sum for the a given criteria with SUMIF for values less then criteria.
Used Functions
Category: Math and trigonometry
Adds the cells specified by a given criteria
Syntax
SUMIF(range, criteria, [sum_range])
Practice SUMIF Formula at Sheetzoom
SUMIF formula at Microsoft help
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The formula matches the given criteria with the lookup range values and return only those that are less then the given criteria, the final sum is obtained by summing up these values.