Percentages are a ratio of 1/100, they're applied multiplicatively. 100% of something is 100/100, so 1. 100% more than something means "add 100%", so a ratio of 2:1, or 200%. We're comparing some value x, and stating that value y is x+100%, so y=x+100%x=200%x=2x, therefore x=y/2=50%y.