Question

Why is an increase 20-40 100% but a decrease of 40-20 is 50%

Answer 1

When 20 changes to 40, it is being increased by 20, which in percent is 100% of the starting number, resulting in a 100% change. This leaves 200% of the starting number left. This is the same thing as multiplying by 2, and 200% is 2!
When 40 changes to 20, it is being decreased by 20, which in percent is only 50% of the starting number. This leaves 50% of the original number left. This is the same thing as dividing by two, and 50% is 1/2!

Hope this helps!

Answer 2

the increase of 20-40 is one hundred percent because it takes one hundred percent of the first number (20) and adds it to itself. but 40-20 is a fifty percent decrease because it takes half (50%) of 40 and subtracts it from itslef