Because I want to know the difference in real percentage, so I subtract with one. 3 is 3 times bigger than 1 but only 200% greater (3/1 - 1 = 2)
3:7=x:100
x = 42.9
If Luffy is 100, then Sanji is 42.9
This is a more impactful and clearer way to see the difference between Luffy and Sanji