What is the percent decrease of 80 to 64

The amount of decrease is  (80 - 64)  =  16

The fraction of decrease is the fraction of the original amount that disappeared.

That fraction is      (16 / 80)  =  1/5  =  0.2

To change a decimal to a percent,
move the decimal point 2 places that way ==> .

                             0.2  =  20%