Percent of change of 18.75 to 18.60

First find the change (or difference) in the two numbers
18.75 - 18.60 = 0.15
Divide this value by the original amount
0.15/18.75 = 0.008
To convert to a percentage, multiply by 100
100 x 0.008 = 0.8%
As it decreases, we put a negative sign in front of the final value
-0.8%

The slope best fit isc. 1/6 mile per hour