1684. The average service life of the motor is 4 years. Estimate from below the probability that this motor will not serve for more than 20 years.
Solution. Let the random variable $X$ be the service life of the motor. From the problem statement, it follows that $E X=4$. We need to estimate from below the probability $P(X<20)$. This probability can be considered as the left side of Markov's inequality with $\xi=20$. Then

$$
P(X<20) \geqslant 1-\frac{E X}{\xi}=1-\frac{4}{20}=0.8
$$