Please help!

Here is the situation:
I need to deternime a weighted average of a set of values for a given
period. I have setup an example below:

period Col 2 Col 3
1 5 100
1 7 200
2 9 100
2 7 200

what would be the best formula to determine the weighted average value of
column 3 weighted on the basis of column 2 by period?

Any help would be appreciated.