Hi there! :D
A change from 20 to 40 is 100% increase because:
If 20 is the number starting up and is the original number, then 20 would equal 100%
40-20=20 An extra 20 is equal to 100%. 20=100%, so it's own number added to itself would make it an increase.
A change from 40 to 20 is a 50% decrease because:
40/2= 20
Half of 40 is 20. Half of a number is equal to 50%.
I hope this helps!
~kaikers