19. A smaller segment $\ell$ with a length of 15 cm is placed on a larger segment $L$ with a length of 40 cm. Find the probability that a point randomly placed on the larger segment will also fall on the smaller segment. It is assumed that the probability of the point falling on segment $\ell$ is proportional to the length of this segment and does not depend on its position on segment $L$.
Solution. Let event $A$ be the point randomly placed on segment $L$ also falls on segment $\ell$.

We will find the probability of event $A$ by applying formula (6):

$$
P(A)=\frac{15}{40}=\frac{3}{8} .
$$