You can use Excel's Solver to arrive at A solution
With the data for example in A1:A10 and the formula
=SUMPRODUCT(A1:A10,B1:B10) in cell C1
Select Solver from the Tools menu
In the window that opens up enter the following
"Set Target Cell" C1 (your sumproduct formula)
"Equal To:" select "Value of: and enter the figure you are seeking
"By Changing Cells" enter B1:B10
"Subject to the Constraints" select Add and enter
"Cell Reference" B1:B10"
Click the drop down in the center and select "bin"
Click Add
Then Click Solve
HTH
PC