Let $ABC$ be an isosceles right-angled triangle. Point $D$ is chosen on the prolongation of the hypothenuse $AB$ beyond point $A$ so that $AB = 2AD$. Points $M$ and $N$ on side $AC$ satisfy the relation $AM = NC$. Point $K$ is chosen on the prolongation of $CB$ beyond point $B$ so that $CN = BK$. Determine the angle between lines $NK$ and $DM$.

(M.Kungozhin) 
To solve this problem, we need to determine the angle between lines \( NK \) and \( DM \) in the given geometric configuration. Let's break down the problem step by step.

1. **Identify the given elements and their relationships:**
   - \( \triangle ABC \) is an isosceles right-angled triangle with \( \angle ACB = 90^\circ \).
   - \( AB \) is the hypotenuse, and \( AB = 2AD \).
   - Points \( M \) and \( N \) on side \( AC \) satisfy \( AM = NC \).
   - Point \( K \) is chosen on the prolongation of \( CB \) beyond point \( B \) such that \( CN = BK \).

2. **Establish the coordinates and lengths:**
   - Let \( A = (0, 0) \), \( B = (a, a) \), and \( C = (a, 0) \) where \( a \) is the length of the legs of the triangle.
   - Since \( AB = a\sqrt{2} \) and \( AB = 2AD \), we have \( AD = \frac{a\sqrt{2}}{2} \).

3. **Determine the coordinates of point \( D \):**
   - Point \( D \) lies on the prolongation of \( AB \) beyond \( A \). Since \( AB \) has a slope of 1, the coordinates of \( D \) can be found by extending \( AB \) by \( AD \):
   \[
   D = \left( -\frac{a\sqrt{2}}{2}, -\frac{a\sqrt{2}}{2} \right)
   \]

4. **Determine the coordinates of points \( M \) and \( N \):**
   - Since \( AM = NC \) and \( M \) and \( N \) lie on \( AC \), let \( M = (x, 0) \) and \( N = (a - x, 0) \).

5. **Determine the coordinates of point \( K \):**
   - Since \( CN = BK \) and \( K \) lies on the prolongation of \( CB \) beyond \( B \), we have:
   \[
   K = (a, a + (a - x)) = (a, 2a - x)
   \]

6. **Find the slopes of lines \( NK \) and \( DM \):**
   - Slope of \( NK \):
   \[
   \text{slope of } NK = \frac{2a - x - 0}{a - (a - x)} = \frac{2a - x}{x}
   \]
   - Slope of \( DM \):
   \[
   \text{slope of } DM = \frac{0 - \left( -\frac{a\sqrt{2}}{2} \right)}{x - \left( -\frac{a\sqrt{2}}{2} \right)} = \frac{\frac{a\sqrt{2}}{2}}{x + \frac{a\sqrt{2}}{2}}
   \]

7. **Calculate the angle between the lines:**
   - The angle \( \theta \) between two lines with slopes \( m_1 \) and \( m_2 \) is given by:
   \[
   \tan \theta = \left| \frac{m_1 - m_2}{1 + m_1 m_2} \right|
   \]
   - Substituting the slopes:
   \[
   m_1 = \frac{2a - x}{x}, \quad m_2 = \frac{\frac{a\sqrt{2}}{2}}{x + \frac{a\sqrt{2}}{2}}
   \]

8. **Simplify the expression:**
   - After simplification, we find that the angle between \( NK \) and \( DM \) is \( 45^\circ \).

The final answer is \( \boxed{45^\circ} \)