help, calculate the value changes.

Q

qye2020

I have one cost field on a simple form, every time user update this cost
field, i need to insert or update the change of cost into a column in the
same table. For example, cost before update is $100, user changes it to
$120, then $20 should be insert into cost_change column. Next time if user
chnage it to $130, then $10 will be persisted.

How to do it in the Access form? Thanks in advance.
 
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