$1\frac{3}{4} : 3,75 = 4х : 15$
4х = 1,75 * 15 : 3,75
4х = 7
$х = \frac{7}{4} = 1\frac{3}{4} = 1,75$

$\frac{1}{2} : 13 = \frac{1}{3}х : 4\frac{1}{3}$
$\frac{1}{3}х = \frac{1}{2} * 4\frac{1}{3} : 13$
$х = \frac{1}{2} * \frac{13}{3} * \frac{1}{13} * 3$
$х = \frac{1}{2} * \frac{1}{1} * \frac{1}{1} * 1$
$х = \frac{1}{2} = 0,5$