Answer :

luana
[tex]x-\ number\\\\ 30\%=\frac{12}{x}\ \ \ \ | multiply\ by\ x\ \ and\ \ \ x \neq 0\\\\ 30\%x=12\\\\ \frac{30}{100}x=12\\\\ 0.3x=12\ \ \ \ | divide\ by\ 0.3\\\\ x=40\\\\ 12\ is\ 30\%\ of\ 40.[/tex]
calculista
let
x-----------> the number

we know that
[tex]30[/tex] % is equal to  [tex] \frac{30}{100} [/tex]
[tex] (\frac{30}{100})*x=12 [/tex]
multiply by 100 both sides 
[tex]30*x=1200[/tex]
divide  by 30 both sides
[tex]x= \frac{1200}{30} \\ x=40 [/tex]

the answer is
the number is [tex]40[/tex]