What is 3.5% of 12.90?

(100%)/(3.5%)=12.90/x where $x$ (what you are looking for) is a fraction of $12.9$ as $3.5$ is a fraction of $100$. The two fractions must give you the same result!
x=(3.5%)/(100%)*12.90=0.035*12.90=0.451