Constant rate of change

what is the constant rate of change in this? 7 days to 4 inches 14 days to 11 inches 21 days to 18 inches 28 days to 25 inches

 

	7	14	21	28
	4	11	18	25
	3	3	3	3
	inches = days +3

 

 

So the constant rate of change is 1 inch per day