Average Based on a Single Criteria
The following formula returns the average for B2:B10, where the corresponding value in A2:A10 equals the value in D2...
=AVERAGEIF(A2:A10,D2,B2:B10)
For Excel 2003 and prior versions, the following array formula can be used...
=AVERAGE(IF(A2:A10=D2,B2:B10))
Note that the formula needs to be confirmed with CONTROL+SHIFT+ENTER. If done correctly, Excel will automatically place curly braces {...} around the formula.
Based on the sample data, the formulas return 80.
Sample Workbook: Download