how can calculet 20%

Say you want to find 20% of $63.00

the percent sign means divide by 100  see it even has a / for divided and two zeros to remind you.

of always means multiply.

so

20% of $63

means

20/100*63

$${\frac{{\mathtt{20}}}{{\mathtt{100}}}}{\mathtt{\,\times\,}}{\mathtt{63}} = {\frac{{\mathtt{63}}}{{\mathtt{5}}}} = {\mathtt{12.6}}$$

And the answer is  $12.60