changing numbers

[metfast]

Hi Please any idea's
In A1 have the number of items (20) in cell B1 i put the number of items sold (2) that then changes cell A1 to 18 items simple (A1-B1).
Q) when i have another sale i type 0 into B1 so i can start new sale, how do i stop A1 from returning to 20 and stay on 18 ect.
Thanks for the help.

 

[mart37]

In the same cell is did not possible.
an example:

Sell
	A	B	C
1	Stock	Sold	Remaining
2	20	2	18
3	18	4	14
4	14
Blad1

Cell Formulas
Range	Formula
C2:C3	C2	=A2-B2
A3:A4	A3	=C2