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Excel worksheet functions: sumif
SUMIF(range, criteria, [sum_range])
Sum up all numbers in the range B1:B100 that are greater or equal to 10. Note that the condition is a string:
=sumif( B1:B100 , ">=10")
Sum up all numbers in the range B1:B100 where the corresponding value in A1:A100 is yes:
=sumif( A1:A100, "yes", B1:B100)
Sum up all numbers in the range B1:B100 where the corresponding value in A1:A100 is not bar. Note that bar itself needs not be put into qutoes:
=sumif( A1:A100, "<>bar", B1:B100)