а) Решим уравнение:
$$y:6,45 = 44,8:17,2$$
$$y = (44,8:17,2) \cdot 6,45$$
$$y = \frac{44,8}{17,2} \cdot 6,45$$
$$y = \frac{448}{172} \cdot \frac{645}{100}$$
$$y = \frac{112}{43} \cdot \frac{129}{20}$$
$$y = \frac{112 \cdot 129}{43 \cdot 20}$$
$$y = \frac{14448}{860} = 16,8$$
б) Решим уравнение:
$$b: \frac{5}{6} = 4: \frac{20}{3}$$
$$b: \frac{5}{6} = 4 \cdot \frac{3}{20}$$
$$b: \frac{5}{6} = \frac{12}{20}$$
$$b: \frac{5}{6} = \frac{3}{5}$$
$$b = \frac{3}{5} \cdot \frac{5}{6}$$
$$b = \frac{15}{30} = \frac{1}{2} = 0,5$$
Ответ: а) 16,8; б) 0,5