\[\boxed{\text{5.376.}}\]
\[\textbf{а)}\ x = 4:\ \]
\[4\ :2 = 2;\ \]
\[2 - 1 = 1 \rightarrow натуральное\ \]
\[число;\ \]
\[1\ :2 = \frac{1}{2};\ \]
\[\frac{1}{2} + 3\frac{1}{2} = 3\frac{2}{2} = 4;\ \]
\[4 \cdot 10 = 40.\ \]
\[\textbf{б)}\ x = 3:\ \]
\[3\ :2 = \frac{3}{2} = 1\frac{1}{2};\ \]
\[1\frac{1}{2} - 1 = \frac{1}{2} \rightarrow не\ натуральное\ \]
\[число;\ \]
\[\frac{1}{2} - \frac{1}{2} = 0;\ \]
\[0 \cdot 25 = 0;\ \]
\[0\ :4 = 0.\ \]
\[\textbf{в)}\ x = 8:\]
\[8\ :2 = 4;\]
\[4 - 1 = 3 \rightarrow натуральное\ \]
\[число;\ \]
\[3\ :2 = \frac{3}{2} = 1\frac{1}{2};\ \]
\[1\frac{1}{2} + 3\frac{1}{2} = 4\frac{2}{2} = 5;\ \]
\[5 \cdot 10 = 50.\ \]
\[\textbf{г)}\ x = 9:\]
\[9\ :2 = \frac{9}{2} = 4\frac{1}{2};\ \]
\[4\frac{1}{2} - 1 = 3\frac{1}{2} \rightarrow не\ натуральное\ \]
\[число;\ \ \]
\[3\frac{1}{2} - \frac{1}{2} = 3;\ \]
\[3 \cdot 25 = 75;\ \]
\[75\ :4 = \frac{75}{4} = 18\frac{3}{4}\text{.\ \ }\]