### Theory:

1. To convert a fraction into a per cent, we multiply the fraction by $$100$$.
2. To convert a ratio into a per cent, we write it as a fraction and multiply it by $$100$$.
3. To convert a decimal into a per cent, we shift the decimal point two places to the right or multiply that by $$100$$ and put the percentage symbol at the end.
4. To convert a per cent into a fraction, we remove the per cent sign($$\%$$) and divide it by $$100$$.
5. To convert a per cent into a ratio, we remove the per cent sign($$\%$$) and form a ratio with the remaining number as the first term and $$100$$ as the second term.
6. To convert a per cent into a decimal, we remove the per cent sign($$\%$$) and shift the decimal points two places to the left.
7. To express $$x$$ as a percentage of $$y$$; percentage $$=$$ $\left[\frac{x}{y}×100\right]%$.
8. If $$x$$$$\%$$of a quantity is $$y$$, then whole quantity $$=$$ $\left[\frac{y}{x}×100\right]%$.