Why do people call a decrease of 40% to 20% a decrease of 20% instead of 50%?

[space-time]

voodoopenguin wrote:
For the same reason when the temperature rises from 5C to 10C some people say the temperature has doubled.

Paul

if you want to have some fun, use 0C and 10C (or any other value) and see how the math works out.

I had someone at work say that 80C was 33% more than 60C. Then I pointed out that if we talk about temperature change when that thing was operational, we should subtract room temperature (usually 20C) so suddenly the 80C item was only 60C above room temperature and 60C object was only 40C above room temperature, so the temperature change was now 50% more in one sample than the other. definitely not 33%.