Answer
$6$
Work Step by Step
The probability that the point is on $\overline{MN}$ is equal to the ratio of the length of $\overline{MN}$ to the length of $\overline{ZB}$.
$\text{P(point on $\overline{MN})$}=\dfrac{MN}{ZB}$
Therefore,
$0.3=\dfrac{MN}{20}\\ \implies MN=6$
