running totals

The first Total formula would be, assuming a header row, =A2*B2.
The rest of the formulas would be =(A3*B3)+C2
This formula can be duplicated down the column.
BTW, If you enter the numbers using Data>Form, The formula will dupicate it self.

Prod Qty Total
A 1 1
B 3 4
C 10 14
D 5 19

Is that what you want? Assume Prod = Col A, Qty = B and Total = C, rows are 1,2, ... You can enter the 1st line as it sits (or in C2 put =B2). C3 would be =C2+B3. CopyC3 and paste in subsequent cells. This is MS Excel but should be same/similar for other spreadsheets (post #2).

Other than an exercise, this really doesn't tell you much. At the end you can only say someting like 'I have 19 inventory items'. It sounds like you are doing somekind of inventory type application. You get the same answer (19 inventory items) by doing =Sum(b2:b4) in some cell.

The more common inventory is:
Prod Qty QtyOnHand Total
A 1 0 1
B 3 5 8
C 10 3 13
D 5 10 15

where d2=b2+c2, d3=b3+c3, etc.

But then it's your application so ... Might post the actual problem definition unless you initial question was correct, total is sum of current line qty and all previous qty values.