Olá,
[tex] \tt \: Decimal{:} \\ \tt\dfrac{12}{30} = \dfrac{ {12}^{ \div 3} }{ {30}^{ \div 3} } = \dfrac{4}{10} = 0{,}4[/tex]
[tex] \tt \: Percentual{:} \\ \tt \: \frac{12}{30} = \frac{4}{10} = \frac{4 \times 10}{10 \times 10} = \frac{40}{100} = 40\%[/tex]
