Original: 15
New: 20
You would want to put new - original / original for percent of increase. In this case, it would be 20 - 15 / 15, which would get you 5 / 15. Simplify: 1 / 3. Now you can change it into decimal, which equals to 0.333333 (With repeat bar on top) and it would equal to 33.3%.