Question

12 is 30 percent of

Answer 1

$x-\ number\\\\ 30\%=\frac{12}{x}\ \ \ \ | multiply\ by\ x\ \ and\ \ \ x \neq 0\\\\ 30\%x=12\\\\ \frac{30}{100}x=12\\\\ 0.3x=12\ \ \ \ | divide\ by\ 0.3\\\\ x=40\\\\ 12\ is\ 30\%\ of\ 40.$

Answer 2

Let
x-----------> the number

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

the answer is
the number is $40$