A 1) \(\frac{a}{48}=\frac{7}{12}\); 2) \(\frac{2}{19}=\frac{4}{y}\); 3) \(\frac{12}{x}=\frac{2}{15}\); 4) \(\frac{25}{4}=\frac{b}{12}\).

Решим пропорции:

1. \(\frac{a}{48}=\frac{7}{12}\)
   \(a = \frac{7 \cdot 48}{12} = \frac{7 \cdot 4 \cdot 12}{12} = 7 \cdot 4 = 28\)
   Ответ: a = 28
2. \(\frac{2}{19}=\frac{4}{y}\)
   \(y = \frac{4 \cdot 19}{2} = 2 \cdot 19 = 38\)
   Ответ: y = 38
3. \(\frac{12}{x}=\frac{2}{15}\)
   \(x = \frac{12 \cdot 15}{2} = 6 \cdot 15 = 90\)
   Ответ: x = 90
4. \(\frac{25}{4}=\frac{b}{12}\)
   \(b = \frac{25 \cdot 12}{4} = 25 \cdot 3 = 75\)
   Ответ: b = 75