Exercises: Random variable. Probability mass function, cumulative distribution function and parameters

A random number app generates integers from $1$ to $4$, all with equal probability. Let $X$ be the number generated.

1. a) Write down the probability function of $X$ in table form.
2. b) Calculate the mean $\mu$.
3. c) Calculate the standard deviation $\sigma$, rounding to two decimal places.

A discrete random variable $X$ takes the values $1$, $2$, $3$ and $4$ with this incomplete probability function:

$x_i$	$1$	$2$	$3$	$4$
$p_i$	$0.15$	$k$	$0.35$	$0.25$

1. a) Calculate the value of $k$.
2. b) Find $P[X \ge 3]$.
3. c) Find $P[X < 2]$.

State, giving reasons, which of the following tables can represent the probability function of a discrete random variable:

• Table 1: values $1$, $2$, $3$ with probabilities $0.3$; $0.4$; $0.2$.

• Table 2: values $1$, $2$, $3$, $4$ with probabilities $0.5$; $0.3$; $0.3$; $-0.1$.

• Table 3: values $1$, $2$, $3$, $4$ with probabilities $0.25$; $0.25$; $0.25$; $0.25$.

In each case, state the condition (or conditions) that are satisfied or violated.

Two coins are flipped and the number of heads obtained, $X$, is counted.

1. a) Write down the sample space of the experiment and deduce the probability function of $X$.
2. b) Calculate the mean $\mu$ and interpret it.
3. c) Calculate $P[X \ge 1]$.

The cumulative distribution function of a discrete random variable $X$ is given by:

$F(0) = 0.2 \qquad F(1) = 0.5 \qquad F(2) = 0.8 \qquad F(3) = 1$

1. a) Calculate $P[X \le 2]$.
2. b) Calculate $P[X = 2]$.
3. c) Calculate $P[X > 1]$.

In a bag for a board game there are $20$ point cards: $6$ cards with a $1$, $4$ cards with a $2$ and $10$ cards with a $3$. A card is drawn at random and its score $X$ is noted.

1. a) Write down the probability function of $X$ in table form.
2. b) Calculate the mean $\mu$ and interpret it: what average score per card can be expected in the long run?
3. c) Calculate the standard deviation $\sigma$, rounding to two decimal places.

The figure shows the cumulative distribution function $F(x)$ of a discrete random variable $X$ that takes the values $0$, $1$, $2$ and $3$.

1. a) Deduce from the graph the probability function of $X$ (the values $p_i = P[X = x_i]$) and write it down in table form.
2. b) Calculate $P[1 \le X < 3]$ using the cumulative distribution function.
3. c) Find the mean of $X$.

A roulette wheel at a fair has $8$ equal sectors with the prizes shown in the figure: one sector with $\$10$, two with $\$5$, three with $\$2$ and two with no prize.

1. a) Write down the probability function of the variable $X$ = "prize obtained in a spin".
2. b) Calculate the average prize per spin, $\mu$.
3. c) Playing costs $\$3$ per spin. Define the variable $G$ = "player's net winnings" (prize minus amount paid), calculate its expected value and decide, giving reasons, whether it is advisable to play.

The PTA of a high school organizes a charity raffle: it sells $200$ tickets at $\$40$ each, with one prize of $\$2\,000$, four prizes of $\$500$ and ten prizes of $\$100$.

1. a) Write down the probability function of the variable $G$ = "net winnings from buying a ticket" (value of the prize minus the $\$40$ it costs).
2. b) Calculate the expected value of $G$.
3. c) Interpret the sign of the result: if the expected value is negative, where does that money go? Does that contradict the charitable purpose of the raffle?

A discrete random variable $X$ takes the values $1$, $2$, $3$ and $4$ with probabilities proportional to the value itself: $P[X = x] = k \times x$.

1. a) Calculate the value of the constant $k$.
2. b) Find the mean $\mu$.
3. c) Verify that the variance is exactly $1$ and deduce the standard deviation.

The waiting time $X$ (in minutes) for the school bus at a stop is uniformly distributed between $0$ and $10$ minutes, that is, its probability density function is constant: $f(x) = k$ for $x \in [0, 10]$ and $f(x) = 0$ elsewhere.

1. a) Calculate the value of $k$ knowing that the area under the probability density function must be $1$.
2. b) Find the probability of waiting between $2$ and $5$ minutes.
3. c) Find the probability of waiting more than $8$ minutes and explain why $P[X = 8] = 0$ even though waiting for exactly $8$ minutes is not impossible.

The time (in minutes) it takes for a server to boot after a restart is a continuous random variable $X$ with probability density function $f(x) = k \times x$ for $x \in [0, 4]$ and $f(x) = 0$ elsewhere, represented in the figure.

1. a) Calculate the value of $k$ knowing that the total area under the probability density function must be $1$.
2. b) Find the probability that the server takes less than $2$ minutes to boot.
3. c) Calculate the median of $X$ (the value $m$ such that $P[X \le m] = \frac{1}{2}$) and the mean $\mu$. Which of the two is greater? What do you think is the reason for this?

Amina is studying the variable $X$ = "number of weekly breakdowns of a photocopier", with this probability distribution:

$x_i$	$0$	$1$	$2$	$3$
$p_i$	$0.1$	$0.3$	$0.4$	$0.2$

She has calculated the mean, $\mu = 1.7$, and then wrote:

«$\sigma^2 = \sum p_i \times x_i^2 - \mu = 3.7 - 1.7 = 2 \implies \sigma = \sqrt{2} \approx 1.41$»

1. a) Check her calculation of $\sum p_i \times x_i^2$ and confirm that it is correct.
2. b) Locate the error in the formula she applied for the variance and correct it.
3. c) Calculate the correct standard deviation.

At a fairground stall, a game is played with two cubic dice: you win $\$240$ if you roll doubles (both dice the same), $\$120$ if the sum is $7$, and nothing in any other case.

1. a) Calculate the probability of winning each prize and the probability of winning nothing. (Note that rolling doubles and having a sum of $7$ are mutually exclusive events; why?)
2. b) Find the expected value of the prize per game and deduce what the price of the stake should be for the game to be fair.
3. c) If the stall charges $\$80$ per game, who does the game favour? How much is a player who plays $100$ games expected to win or lose?

A discrete random variable $X$ takes the values $0$, $1$, $2$ and $4$, and its cumulative distribution function satisfies:

$F(0) = 0.1 \qquad F(1) = 0.5 \qquad F(2) = a \qquad F(4) = 1$

1. a) Knowing that the mean of the distribution is $\mu = 2.2$, set up the equation that allows you to find $a$ and calculate its value.
2. b) Write down the complete probability function of $X$.
3. c) Calculate the variance and standard deviation of $X$, rounding to two decimal places.

Let $X$ be a discrete random variable with mean $\mu$ and standard deviation $\sigma$, and let $Y = a \times X + b$ be a new variable obtained by applying a linear transformation to $X$.

1. a) Prove, from the definition of the mean, that $\mu_Y = a \times \mu + b$.
2. b) Prove that $\sigma_Y = |a| \times \sigma$. Why does the constant $b$ not affect the spread?
3. c) Application: in an exam, the marks have a mean of $6.2$ and a standard deviation of $1.4$. The teacher decides to transform each mark $X$ into $Y = 0.9 \times X + 1$. Calculate the new mean and the new standard deviation.