$$\frac{x}{15} = \frac{1}{5}$$
$$x = \frac{15 \cdot 1}{5} = 3$$
$$\frac{10}{x} = \frac{5}{12}$$
$$x = \frac{10 \cdot 12}{5} = 24$$
$$\frac{8}{x} = \frac{4}{4}$$
$$x = \frac{8 \cdot 4}{4} = 8$$
$$\frac{18}{27} = \frac{x}{3}$$
$$x = \frac{18 \cdot 3}{27} = 2$$
Ответ: а) 3, б) 24, в) 8, г) 2