SUMIF(range,criteria[,actual_range])
SUMIF function sums the values in the given range that meet the given criteria. If actual_range is given, SUMIF sums the values in the actual_range whose corresponding components in range meet the given criteria.
Let us assume that the cells A1, A2, ..., A5 contain numbers 23, 27, 28, 33, and 39. Then
SUMIF(A1:A5,"<=28") equals 78.
SUMIF(A1:A5,"<28") equals 50.
In addition, if the cells B1, B2, ..., B5 hold numbers 5, 3, 2, 6, and 7 then:
SUMIF(A1:A5,"<=27",B1:B5) equals 8.