Percentage is a unit for expressing a numerical relationship as a fraction of 100. One percent is one hundredth part of a whole. The symbol "%" is used for percentage.
$$1\% = \frac{1}{100}=0.01$$
One per mille (permille) is one thousandth part of a whole. The symbol "‰" is used for per mille.
$$1‰ = \frac{1}{1000}=0.001$$
What percentage does number a represent of number b:
$$\frac{a}{b}\times 100\%$$
Find n% of number a:
$$n\%\times a = \frac{n\times a}{100}$$
Find the number of which n% is b:
$$\frac{b}{n\%}=\frac{b\times 100}{n}$$
What percentage is the change a from the initial value A:
$$\frac{a}{A}\times 100\%$$