б) \frac{5}{12} b при b = \frac{1}{5}, b = \frac{5}{12}, b = \frac{6}{5}, b = \frac{84}{25}, b = 0.

Найдем значение выражения:

1. \(b = \frac{1}{5}\):
   $$\frac{5}{12} \cdot \frac{1}{5} = \frac{5 \cdot 1}{12 \cdot 5} = \frac{1}{12}$$
2. \(b = \frac{5}{12}\):
   $$\frac{5}{12} \cdot \frac{5}{12} = \frac{25}{144}$$
3. \(b = \frac{6}{5}\):
   $$\frac{5}{12} \cdot \frac{6}{5} = \frac{5 \cdot 6}{12 \cdot 5} = \frac{30}{60} = \frac{1}{2}$$
4. \(b = \frac{84}{25}\):
   $$\frac{5}{12} \cdot \frac{84}{25} = \frac{5 \cdot 84}{12 \cdot 25} = \frac{420}{300} = \frac{7}{5} = 1\frac{2}{5}$$
5. \(b = 0\):
   $$\frac{5}{12} \cdot 0 = 0$$

Ответ: Если \(b = \frac{1}{5}\), то \(\frac{1}{12}\); если \(b = \frac{5}{12}\), то \(\frac{25}{144}\); если \(b = \frac{6}{5}\), то \(\frac{1}{2}\); если \(b = \frac{84}{25}\), то \(1\frac{2}{5}\); если \(b = 0\), то 0.