In this exercise, we study some characteristic quantities of the operation of parking lots in a city. Throughout the exercise, probabilities will be given with a precision of $10 ^ { - 4 }$.
Parts A, B, and C are independent
Part A - Waiting time to enter an underground parking lot
The waiting time is defined as the time that elapses between the moment the car arrives at the parking entrance and the moment it passes through the parking entrance barrier. The following table presents observations made over one day.
| Waiting time in minutes | $[ 0 ; 2 [$ | $[ 2 ; 4 [$ | $[ 4 ; 6 [$ | $[ 6 ; 8 [$ |
| Number of cars | 75 | 19 | 10 | 5 |
- Propose an estimate of the average waiting time for a car at the parking entrance.
- We decide to model this waiting time by a random variable $T$ following an exponential distribution with parameter $\lambda$ (expressed in minutes). a. Justify that we can choose $\lambda = 0.5 \mathrm {~min}$. b. A car arrives at the parking entrance. What is the probability that it takes less than two minutes to pass through the barrier? c. A car has been waiting at the parking entrance for one minute. What is the probability that it passes through the barrier in the next minute?
Part B - Duration and parking rates in this underground parking lot
Once parked, the parking duration of a car is modeled by a random variable $D$ that follows a normal distribution with mean $\mu = 70 \mathrm {~min}$ and standard deviation $\sigma = 30 \mathrm {~min}$.
- a. What is the average parking duration for a car? b. A motorist enters and parks in the parking lot. What is the probability that their parking duration exceeds two hours? c. To the nearest minute, what is the maximum parking time for at least $99 \%$ of cars?
- The parking duration is limited to three hours. The table gives the rate for the first hour and each additional hour is charged at a single rate. Any hour started is charged in full.
| \begin{tabular}{ c } Parking |
| duration |
& Less than 15 min & Between 15 min and 1 h &
\hline Rate in euros & Free & 3.5 & $t$ \hline \end{tabular}
Determine the rate $t$ for the additional hour that the parking manager must set so that the average parking price for a car is 5 euros.
Part C - Waiting time to park in a city center parking lot
The parking duration of a car in a city center parking lot is modeled by a random variable $T ^ { \prime }$ that follows a normal distribution with mean $\mu ^ { \prime }$ and standard deviation $\sigma ^ { \prime }$. It is known that the average parking time in this lot is 30 minutes and that $75 \%$ of cars have a parking time less than 37 minutes. The parking manager aims for the objective that $95 \%$ of cars have a parking time between 10 and 50 minutes. Is this objective achieved?