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BAC2 ss · Mathematics
Chance and probability · Probability

Laplace's Rule

You toss a coin, spin a spinner, draw a card: in all these experiments, if they are set up properly, every outcome has the same chance as the others. When that happens, calculating a probability comes down to counting: how many outcomes you are interested in, out of how many there are in total. That is all there is to Laplace's Rule. The challenge in 2nd Bachillerato is not the formula, which you already know, but counting correctly when the possible or favourable cases number in the thousands or millions.

Before you start, make sure you know: Counting techniques (1st Bachillerato)
Before you start, make sure you know: Combinatorics (1st Bachillerato)

What "at random" means: equally likely events

Comparison between a spinner divided into four equal sectors with equally likely outcomes and another with unequal sectors where outcomes are not equally likely. 12 43 spinner: 4 equal sectors large sector spinner: different sectors → NOT equally likely
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A spinner divided into $12$ equal sectors, numbered from $1$ to $12$, is used. Calculate the probability that when the spinner is spun, the pointer stops on: a) A number…

A spinner divided into $12$ equal sectors, numbered from $1$ to $12$, is used. Calculate the probability that when the spinner is spun, the pointer stops on:

  1. a) A number divisible by $3$.
  2. b) A number that is a perfect square (such as $1, 4, 9$).
  3. c) A number greater than $2$ and less than $7$.
n=12 1 2 3 4 5 6 7 8 9 10 11 12 Ω = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}
Solve
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We toss two fair coins into the air simultaneously. a) Write the sample space of the experiment. b) What is the probability of getting exactly one head? c) What is the…

We toss two fair coins into the air simultaneously.

  1. a) Write the sample space of the experiment.
  2. b) What is the probability of getting exactly one head?
  3. c) What is the probability of getting at least one tail?
Solve

Laplace's Rule: favourable cases over possible cases

Conceptual diagram of Laplace's rule showing probability as the ratio between the number of favourable cases and the number of possible cases in an equally likely sample space. 3 favourable 8 possible P(A) = 3/8
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In a role-playing game, a player rolls an eight-sided die (octahedron) with faces numbered $1$ to $8$. If the die is fair, calculate the following probabilities using…

In a role-playing game, a player rolls an eight-sided die (octahedron) with faces numbered $1$ to $8$. If the die is fair, calculate the following probabilities using Laplace's rule:

  1. a) Getting an even number.
  2. b) Getting a number greater than $5$.
  3. c) Getting a prime number.
8 2 4 7 Sample space (Ω) 1 2 3 4 5 6 7 8 8 equally likely outcomes
Solve
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One card is drawn at random from a Spanish deck of $40$ cards. The deck has $4$ suits (oros [coins], copas [cups], espadas [swords] and bastos [clubs]) with $10$ cards…

One card is drawn at random from a Spanish deck of $40$ cards. The deck has $4$ suits (oros [coins], copas [cups], espadas [swords] and bastos [clubs]) with $10$ cards each: from $1$ (ace) to $7$, plus the sota (S) [jack], the caballo (C) [knight] and the rey (R) [king]. Determine the probability of the following events:

  1. a) The card is an ace.
  1. b) It is a cup (copa).
  1. c) It is the king of coins (rey de oros).
Grid with the 40 cards of the Spanish deck: four rows (coins, cups, swords and clubs) and ten columns (1 to 7, jack, knight and king)1234567SCROrosCopasEspadasBastos
Solve
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At a board game festival, two fair six-sided dice (with faces from 1 to 6) are used. A player rolls both dice at the same time. The table shows the sum of the scores for…

At a board game festival, two fair six-sided dice (with faces from 1 to 6) are used. A player rolls both dice at the same time. The table shows the sum of the scores for each possible outcome (rows: die 1; columns: die 2).

Two-way table with the sums of the scores of two dice, each numbered from 1 to 6 Die 2 Die 1 123456 123456 234567 345678 456789 5678910 67891011 789101112

Calculate the probability that the sum of the scores obtained is a prime number greater than $5$.

Solve

Laplace with combinatorics: counting without listing

When the possible cases number in the thousands (hands of cards, passwords, teams chosen from a large group), they are not listed one by one: permutations with selection, permutations and combinations are used to count them directly.

Comparative diagram for choosing between combinations when order does not matter and arrangements when order does matter. order does NOT matter choose 5 from 15 combinations order DOES matter 4-digit PIN arrangements
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In a glass urn for a raffle at a tech store, there are $12$ white balls and $8$ black balls. Three balls are drawn at random, one by one and without replacement.…

In a glass urn for a raffle at a tech store, there are $12$ white balls and $8$ black balls. Three balls are drawn at random, one by one and without replacement. Applying Laplace's rule to the sample space of equiprobable elementary events, determine the probability that the three drawn balls are of the same colour.

Probability: Laplace's Rule Draw of 3 balls without replacement 12 White, 8 Black 3 draws without replacement ? ? ? Same colour? or P = ?
Solve
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In an eSports club, a team of 5 players is to be selected for a national tournament. 15 candidates have entered the selection process: 8 strategy specialists and 7…

In an eSports club, a team of 5 players is to be selected for a national tournament. 15 candidates have entered the selection process: 8 strategy specialists and 7 reflex specialists. The tournament rules require the team to have at least 3 strategy specialists and at least 1 reflex specialist. If the board chooses the 5 members at random from among all the candidates, calculate the probability that the team strictly complies with the rules.

8 Estrategia 7 Reflejos Choose 5 at random Equipos válidos (≥ 3 Estrategia y ≥ 1 Reflejos) 3 Est. + 2 Ref. OR 4 Est. + 1 Ref.
Solve
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Using the letters of the word $ALGORITMO$, all possible permutations are formed. If one of these arrangements is chosen at random, calculate the probability that the…

Using the letters of the word $ALGORITMO$, all possible permutations are formed. If one of these arrangements is chosen at random, calculate the probability that the resulting word begins and ends with a consonant, and also that all the vowels appear together somewhere in the string.

Solve

Geometric probability: when the cases are continuous

Illustration of geometric probability showing a favourable region bounded within a total continuous area or length. 20 cm square circle P(outside the circle) = (area of square − area of circle) / area of square
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An aiming video game consists of throwing a virtual dart at a square target of side length $20\text{ cm}$ that has an inscribed orange circle. The program guarantees…

An aiming video game consists of throwing a virtual dart at a square target of side length $20\text{ cm}$ that has an inscribed orange circle. The program guarantees that the dart always hits somewhere on the square at random (equally likely). Calculate the probability that the dart hits outside the orange circle. (Consider $\pi \approx 3,14$).

r 20 cm
Solve
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A graphic design application generates a random pattern by placing a point $P$ inside a circle of radius $R = 10 \text{ cm}$. A square of side length $L = 8 \text{ cm}$…

A graphic design application generates a random pattern by placing a point $P$ inside a circle of radius $R = 10 \text{ cm}$. A square of side length $L = 8 \text{ cm}$ is inscribed in the centre of the circle. The application's algorithm considers it a "visual hit" if point $P$ falls inside the square or in the region of the circle above the horizontal diameter but outside the square. Using the concept of geometric probability (extension of Laplace's Rule for areas), calculate the probability of getting a "visual hit" in one attempt.

R = 10 L = 8 P
Solve

Summary

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