两列摘要（加权均值） - jqGrid

Question

I need to calculate the weighted mean of two columns i.e.SUM(COL1*COL2)/SUM(COL2) in jqGrid----i'm using jqSuite php by trirand.

I am able to take the two columns and find the sum of its product by

    psuedoCOL=>array(col1*col2+>"SUM")

psuedoCOL stores the product of the two columns rowwise

I just don't know how to access the sum in psuedoCOL to generate my weighted mean