$12x - \frac{3}{4} = 0$
$12x = \frac{3}{4}$
$x = \frac{3}{4} : 12$
$x = \frac{3}{4} * \frac{1}{12} = \frac{1}{4} * \frac{1}{4}$
$x = \frac{1}{16}$

$0,8 + \frac{1}{4}x = 0$
$\frac{1}{4}x = -0,8$
$x = -0,8 : \frac{1}{4}$
x = −0,8 * 4
x = −3,2

$0,7x + \frac{1}{5} = 0$
$0,7x = -\frac{1}{5}$
$x = -\frac{1}{5} : \frac{7}{10}$
$x = -\frac{1}{5} * \frac{10}{7}$
$x = -\frac{2}{7}$

$\frac{2}{5} - 10x = 0$
$-10x = -\frac{2}{5}$
x = 0,4 : 10
x = 0,04