Let $ABC$ be a triangle and let $P$ be a point in its interior. Suppose $ \angle B A P = 10 ^ { \circ } , \angle A B P = 20 ^ { \circ } , \angle P C A = 30 ^ { \circ } $ and $ \angle P A C = 40 ^ { \circ } $. Find $ \angle P B C $.
1. Given the angles in the triangle \( \triangle ABC \) with point \( P \) inside it, we have:
   \[
   \angle BAP = 10^\circ, \quad \angle ABP = 20^\circ, \quad \angle PCA = 30^\circ, \quad \angle PAC = 40^\circ
   \]

2. First, we find \( \angle APB \):
   \[
   \angle APB = 180^\circ - \angle BAP - \angle ABP = 180^\circ - 10^\circ - 20^\circ = 150^\circ
   \]

3. Next, we find \( \angle BPC \):
   \[
   \angle BPC = 180^\circ - \angle APB - \angle PCA = 180^\circ - 150^\circ - 30^\circ = 100^\circ
   \]

4. Using the Law of Sines in \( \triangle ABP \):
   \[
   \frac{AP}{\sin 20^\circ} = \frac{BP}{\sin 10^\circ} = \frac{c}{\sin 150^\circ}
   \]
   Since \( \sin 150^\circ = \sin 30^\circ = \frac{1}{2} \), we have:
   \[
   \frac{c}{\sin 150^\circ} = 2c
   \]
   Therefore:
   \[
   AP = 2c \sin 20^\circ, \quad BP = 2c \sin 10^\circ
   \]

5. Using the Law of Sines in \( \triangle APC \):
   \[
   \frac{AP}{\sin 30^\circ} = \frac{CP}{\sin 40^\circ}
   \]
   Since \( \sin 30^\circ = \frac{1}{2} \), we have:
   \[
   \frac{AP}{\frac{1}{2}} = 2AP = \frac{CP}{\sin 40^\circ}
   \]
   Therefore:
   \[
   CP = 2AP \sin 40^\circ = 4c \sin 20^\circ \sin 40^\circ
   \]
   Using the double-angle identity \( \sin 2\theta = 2 \sin \theta \cos \theta \), we get:
   \[
   CP = c (2 \cos 20^\circ - 1)
   \]

6. Let \( \angle PBC = x \). Using the Law of Sines in \( \triangle BPC \):
   \[
   \frac{CP}{\sin x} = \frac{BP}{\sin (80^\circ - x)}
   \]
   Substituting the values:
   \[
   \frac{c (2 \cos 20^\circ - 1)}{\sin x} = \frac{2c \sin 10^\circ}{\sin (80^\circ - x)}
   \]
   Simplifying, we get:
   \[
   \frac{2 \cos 20^\circ - 1}{\sin x} = \frac{2 \sin 10^\circ}{\sin (80^\circ - x)}
   \]

7. Using the identity \( \sin (80^\circ - x) = \cos (10^\circ + x) \), we have:
   \[
   \frac{2 \cos 20^\circ - 1}{\sin x} = \frac{2 \sin 10^\circ}{\cos (10^\circ + x)}
   \]

8. After simplification and factorization, we get:
   \[
   2 \cos 20^\circ \cos (10^\circ + x) = 2 \sin 10^\circ \sin x + \cos (10^\circ + x) = \cos (x - 10^\circ)
   \]

9. Solving the equation \( \cos (30^\circ + x) = 0 \):
   \[
   30^\circ + x = 90^\circ \implies x = 60^\circ
   \]

The final answer is \( \boxed{60^\circ} \)