\[\boxed{\mathbf{190.}}\]
\[\textbf{а)}\ Р = \frac{1}{4}.\]
\[\textbf{б)}\ Р = \frac{S_{\text{AMD}}}{S_{\text{ABCD}}} = \frac{\frac{1}{2} \cdot a \cdot \frac{2}{3} \cdot a}{a^{2}} = \frac{1}{3}.\]
\[\textbf{в)}\ \frac{\text{CM}}{\text{MD}} = \frac{m}{n};\]
\[Р = \frac{S_{\text{AMD}}}{S_{\text{ABCD}}} = \frac{\frac{1}{2} \cdot a \cdot \frac{m}{m + n} \cdot a}{a^{2}} =\]
\[= \frac{n}{2 \cdot (m + n)}.\]