To find a weighted average, follow these steps: In a new worksheet, type the following data: A1: Cost B1: Cases. A2: $.20 B2: 10. A3: $.30 B3: 40. Type the formula below in any blank cell (it is not necessary to type this formula as an array): =SUMPRODUCT(A2:A3,B2:B3)/SUM(B2:B3)