In a convex quadrilateral $ABCD$ we are given that $\angle CAD=10^\circ$, $\angle DBC=20^\circ$, $\angle BAD=40^\circ$, $\angle ABC=50^\circ$. Compute angle $BDC$.
1. **Extend \(AD\) and \(BC\) to meet at a point \(E\)**:
   - This construction helps us to use external angles and properties of triangles to find the desired angle.

2. **Analyze triangle \(ABE\)**:
   - Given \(\angle BAD = 40^\circ\) and \(\angle ABC = 50^\circ\), we can deduce that \(\angle ABE = 90^\circ\) since the sum of angles in a triangle is \(180^\circ\).

3. **Determine \(\angle BDA\)**:
   - Since \(\angle BAD = 40^\circ\) and \(\angle CAD = 10^\circ\), we have:
     \[
     \angle BDA = 180^\circ - \angle BAD - \angle CAD = 180^\circ - 40^\circ - 10^\circ = 130^\circ
     \]

4. **Determine \(\angle ACE\)**:
   - Given \(\angle DBC = 20^\circ\) and \(\angle ABC = 50^\circ\), we have:
     \[
     \angle ACE = 180^\circ - \angle ABC - \angle DBC = 180^\circ - 50^\circ - 20^\circ = 110^\circ
     \]

5. **Use trigonometric relationships in triangles**:
   - In \(\triangle ABE\), since \(\angle ABE = 90^\circ\), we can use trigonometric ratios:
     \[
     BE = AB \cdot \sin(40^\circ) \quad \text{and} \quad AE = AB \cdot \cos(40^\circ)
     \]

6. **Calculate \(CE\) and \(DE\)**:
   - In \(\triangle ACE\), since \(\angle ACE = 110^\circ\), we have:
     \[
     CE = AE \cdot \cot(80^\circ) = AB \cdot \cos(40^\circ) \cdot \cot(80^\circ)
     \]
   - In \(\triangle BDE\), since \(\angle BDE = 20^\circ\), we have:
     \[
     DE = BE \cdot \tan(20^\circ) = AB \cdot \sin(40^\circ) \cdot \tan(20^\circ)
     \]

7. **Find the ratio \(\frac{DE}{CE}\)**:
   - Using the trigonometric identities:
     \[
     \frac{DE}{CE} = \frac{AB \cdot \sin(40^\circ) \cdot \tan(20^\circ)}{AB \cdot \cos(40^\circ) \cdot \cot(80^\circ)} = \frac{\sin(40^\circ) \cdot \tan(20^\circ)}{\cos(40^\circ) \cdot \cot(80^\circ)}
     \]
   - Simplifying using \(\tan(80^\circ) = \cot(10^\circ)\):
     \[
     \frac{DE}{CE} = \tan(20^\circ) \cdot \tan(40^\circ) \cdot \tan(80^\circ) = \tan(60^\circ) = \sqrt{3}
     \]

8. **Determine \(\angle CDE\)**:
   - Since \(\frac{DE}{CE} = \sqrt{3}\), \(\angle CDE = 30^\circ\).

9. **Calculate \(\angle BDC\)**:
   - Using the sum of angles in a triangle:
     \[
     \angle BDC = 180^\circ - \angle BDA - \angle CDE = 180^\circ - 130^\circ - 30^\circ = 20^\circ
     \]

The final answer is \(\boxed{40^\circ}\)