Weighted average till sum of exposure reaches a threshold
 

Dear Guru,
I would like to calculate the weighted average of rating (weighted by exposure) till the sum of exposure reaches certain value. please see table below:

Buyer Exposure Rating
1 900.00 6
2 675.00 6
3 675.00 6
4 675.00 6
5 450.00 6
6 900.00 5
7 675.00 5
8 675.00 5
9 675.00 5
10 675.00 5
11 675.00 5
12 675.00 4


Total Exposure 4,050.00

I would like to first sort the ratings from highest to lowest, then calculate the weighted average of Rating (weighted by exposure) till sum of exposure is equals to Total exposure value (in this example 4,050. I will sumproduct the first 5.25 buyers to reach my goal. if this is too complex, sumproduct the first 6 buyers will also do the job.

Thanks in advance for your kind help.

Maryann