You have 80 pens for example, and it's all what you have, so it's 100% of your owns; you used 20 and still 60 and you want to find he percentage of what it rest!
So you make
100% _______80
X _______60
you will have 100% * 60 = X * 80 so X = 100% * 60/ 80
X= 75%
