Explain why a change from 20 to 40 is a 100% increase, but a change from 40 to 20 is a 50% decrease

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