Exercises: Games of chance and decisions

Three friends play a game by tossing two coins: Adrián wins if two heads come up, Blanca if two tails come up, and Carlos if one head and one tail come up.

a) Write down the sample space.

b) Calculate the probability of each person winning.

c) Is it a fair game? Who has the advantage?

In a game, you win if you roll a $5$ or a $6$ when rolling a die, and you lose in any other case.

a) What is the probability of winning?

b) If you play $30$ times, how many wins would you expect?

A spinner is divided into two equal halves, one red and one blue. Two players each bet on a color and win $1$ point for a correct guess. Is it a fair game? Why?

In a game you bet $1$ counter on a number from $1$ to $6$ and roll a die: if your number comes up, you get $6$ counters (yours plus another $5$); if not, you lose your counter.

a) What is the probability of winning?

b) If you play $60$ times, how many times would you expect to win? What would be your overall balance of counters?

c) Is the game fair? Why?

Elsa and Bruno roll two dice: Elsa wins if the total is even and Bruno if it is odd.

a) Is it a fair game? Calculate each person's probability of winning.

b) What if, instead of adding, they multiply the results and Elsa continues betting on even? Calculate the new probabilities and explain who has the advantage.

At a carnival, a booth offers this game: you bet $\text{RD\$}\ 1$ on a number from $1$ to $6$ and a die is rolled; if your number comes up, you get $\text{RD\$}\ 5$ back (your pesos plus four more); otherwise, you lose your pesos.

a) What is the probability of winning?

b) If you play $60$ times, what would your expected balance be?

c) Is it a fair game? If not, how much should the prize pay to make it fair?

Three counters are flipped: one with green and red sides, another with green and blue, and another with red and blue. Player $1$ wins if any two counters show the same color; Player $2$ wins if all three colors are different.

a) Using a tree diagram, calculate the probability of winning for each player.

b) Is it a fair game?

c) Propose a change in the rules or in the points system to make it a fair game.

At the fair booth shown in the figure, spinning the wheel costs $\text{RD\$}\ 120$.

a) Calculate the probability of each outcome (no prize, bracelet, and stuffed animal).

b) If you play $100$ times, costing $\text{RD\$}\ 12\,000$, what total value of prizes would you expect to receive?

c) Is it worth playing? Who does the game favor?

Carla and Jesús play with two dice whose faces are the first six prime numbers ($2$, $3$, $5$, $7$, $11$, $13$). Jesús proposes: "If the product is $49$, you win; if it is $6$, I win." Carla does not accept.

a) How many pairs give a product of $49$? And a product of $6$?

b) Why do you think Carla does not accept? Calculate the probabilities.

c) Propose a product for each player that makes the game fair.

State whether these statements about lotteries and chance are true or false, giving reasons:

a) Since the jackpot almost never lands in my town, I have a higher probability of winning if I buy the ticket in another town.

b) The number $00\,027$ is less likely to be drawn than $23\,568$.

c) If the ending in $5$ has come up many times in recent draws, it is now less likely to come up.

d) If a coin has landed on heads $6$ times in a row, on the next toss tails is more likely.

Two friends are playing a game drawing a card from a Spanish deck of cards: you win if a face card is drawn and your friend wins if a card from the suit of bastos is drawn.

a) Calculate the probability of each person winning.

b) Who has the advantage?

c) If you want the game to be fair without changing the events, how many points should each person award for a win? Explain your proposal.

A ball is drawn from a bag containing $4$ red balls and $4$ green balls: if it is red, you win $3$ points; if it is green, your friend wins $5$ points. After each round, the ball is put back in the bag.

a) What is the probability of each person scoring?

b) If you play $40$ rounds, what score would you expect?

c) Is it a fair game? Why?

A game uses the three spinners shown in the figure. Each player chooses a spinner, both chosen spinners are spun, and whoever gets the higher number wins.

a) If the green spinner and the blue spinner face each other, who wins and with what probability?

b) If the blue spinner and the red spinner face each other, who wins and with what probability?

c) If the red spinner and the green spinner face each other, who wins and with what probability?

d) If your opponent chooses a spinner first, what strategy would you follow? Is there a spinner that is "the best"? Explain the result.

Three special dice have these faces:

• Red die: $4$, $4$, $4$, $4$, $4$, $1$

• Green die: $2$, $2$, $2$, $5$, $5$, $5$

• Blue die: $3$, $3$, $3$, $3$, $3$, $6$

Two players each choose a die and roll them; the higher number wins.

a) Calculate the probability that the red die beats the green die.

b) Calculate the probability that the green die beats the blue die.

c) Calculate the probability that the blue die beats the red die.

d) If your opponent chooses a die first, which die would you choose in each case? Does the result surprise you? Comment on it.

A spinner has $6$ equal sectors: $2$ red and $4$ blue. Two players bet: player A on red and player B on blue.

a) What are the probabilities of winning for each player?

b) If in each game the winner takes the points that both bet, suggest how many points each player should bet to make the game fair. Justify your proposal.

c) Design another division of sectors and bets that also makes a fair game.

In a video game, a chest costs $25$ gold coins and contains: a common item (worth $10$ coins) with probability $0.7$; a rare item (worth $50$ coins) with probability $0.25$; or an epic item (worth $300$ coins) with probability $0.05$.

a) If you buy $100$ chests, how many items of each type would you expect to get?

b) What would be the total expected value of these items? And the total expenditure?

c) Is it worth buying chests in the long run? Justify your answer.