increasing 20 to 40 is the same as increasing 20 by 20 (or itself), so it is a 100% increase. Decreasing 40 to 20 is the same as decreasing 40 by 1/2 or by 20. So it is a 50% increase.
when going from 20 to 40, the number is being doubled, and when youre going from 40 to 20 you are cutting the number in half.
hope this helped:)