Pls. help me

Hi! Please anyone guide me as to how to apply "average clause" for the "loss of stock" in the following question:-

A fire occurred in the business premises of M/s Aggarwal & Sons on 15th October, 2008. From the following particulars ascertain the loss of stock and prepare a claim for insurance:-

Stock(1-01-2007) ------------------------------------------------------- Rs.45900

Purchase(from 1-01-2007 to 31-12-2007) -----------------------Rs.183000

Sales(from 1-01-2007 to 31-12-2007)-----------------------------Rs.270000

Stock(31-12-2007)-------------------------------------------------------Rs.40500

Purchase(from 1-01-2008 to 14-10-2008)------------------------Rs.220500

Sales(from 1-01-2008 to 14-10-2008)-----------------------------Rs.225000

           The stocks were always valued at 90% of cost. The stock saved from fire was Rs.27000. The amount of policy was Rs.94500. There was an 'average clause' in the polilcy.