heloo
if i have 5 columns like these

suppose that this is a sell and buy stocks forum
the first column is company number
the second column is company name
the third column is for quantity
the forth is for buy price
the fifth column is for sell price
as shwon belo

1 microsoft 100 50
2 sakhr
3 intel
1 microsoft 100 60
1 microsoft 100 (this is blank cell) 40
1 microsoft 100 40

and i want to average the price of the stock (for only the numer one company (microsoft) )
so the result will be
((100*50+100*60)/(100+100))=55 (is the average price of the 200 stock)(downto the forth row)

then i sell 100 (in the fifth row)
so the result is
100 stock with average price 55
then i buy 100 with price 40
so the final result will be (100*55+100*40)/(100+100)=47.5
how can i do that in excel???