Need Help !!! How to multiply 2 adjacent columns with blanks

Hi,

i'm newbie here, hope u can help me guys...thanks in advance..

How can i multiply 2 adjacent columns shown below:

A B C Total

1 Item Unit Price Qty
2 Product 1 9.00 300 2,700
3 Product 1 <blank same price> <blank> <s/b 2,700 also> if both col B or C are blank s/b empty
4 Product 1 <blank same price> 200 <s/b 9.00 x 200 = 1,800> if col.B or C is blank then use previous value above
5 Product 1 <blank same price> <blank> <s/b 9.00 x 200 = 1,800>
6 Product 1 10.00 100 <s/b 10.00 x 100 = 1,000>

i attached the file..

Thanks.

Chico