That's not how the math works...you seem to have done a temperature conversion and ignore that Celsius and Fahrenheit place their O's at different places
+1.5 degrees C is about +3 degrees F but 0 degrees C is 32 degrees F.
+1.5 degrees C is about +3 degrees F but 0 degrees C is 32 degrees F.
If not: We are not talking of the temperature 1.5°C. We are talking about a temperature difference of 1.5°C.
What you actually want to do is find how much Fahrenheit _delta_ is equivalent to a 1.5C delta. Try calculating the Fahrenheit value for 20C, and then for 21.5C, and then subtract.
Or you can look at the equation you're using, where the slope is a clear and constant 9/5. 9/5*1.5 = 2.7