trigger based in cell value change

[ccruz]

Hi,
i´d like to know if there´s a way to, programmatically or via a formula,
check a cell if value is changed and add a percentage of the difference until
reach the new value
ex- in the collum C line 1 I have 3500 and if in the next line the
value changed to 3000
that will be the trigger to
in the column D line 2 show 3500 - (3500- 3000 *20%) in
line 3 , 4 , 5 etc same until reach the 3000 ( value of new entrance) -

the trigger should be maintained until new change occur in column C

can you help ?
Tks