Conditional Probability

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This is level 1: a mixed selection of conditional probability questions. You will be awarded a trophy if you get at least 9 answers correct and you do this activity online.

In a bookstore, there are two sections: fiction and non-fiction. In the fiction section, 70% of the books have hard covers, while in the non-fiction section, only 20% of the books have hard covers. If a customer picks a book at random and it has a hard cover, what is the probability that it is from the non-fiction section? Assume that there are an equal number of books in the fiction and non-fiction sections.

Give your answer to three significant figures.

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40 Scouts went hiking. 20 got lost, 20 got blisters, and 10 got both lost and blisters. Find the probability that a randomly selected Scout got blisters, given that they were not lost.

Give your answer as a fraction.

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Every family in Happyland has either has a car or a motor scooter or both. 74% of the families have a car. 76% of the families have a scooter. A family is selected at random and it is found that they have a car. Find the probability they also have a scooter.

Give your answer as a fraction.

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Box A contains 7 red and 9 blue cubes, and box B contains 12 red and 15 blue cubes. Finn selects a box at random and takes a cube from that box. Given that the cube is red, what is the probability that it came from box B?

Give your answer as a fraction.

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What is the probability that it was machine B that broke down if at least one of the independent machines broke down today, given machine A has a 8% chance and machine B has a 13% chance of breaking down on any given day?

Give your answer to three significant figures.

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Each afternoon the probability my cat sleeps is 0.7 and the probability that my dog sleeps is 0.6. The probability that the dog sleeps given that the cat is sleeping is 0.9. Find the probability that both sleep in the afternoon.

Give your answer to three significant figures.

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Tin A contains 7 red balls and 9 green balls. Tin B contains 12 red balls and 15 green balls. A dice is thrown and if the score is less than 3 a ball is selected from tin A; otherwise a ball is selected from tin B. Given that the ball selected was red, calculate the probability that it came from tin B.

Give your answer as a fraction.

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The probability that I drop and brake my phone when I visit a coffee shop is 0.05. Today I visited two coffee shops and broke my phone in one of them. What is the probability that it was the first shop where the accident occurred?

Give your answer to three significant figures.

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The probability that it is cloudy on a particular day is 0.4. The probability that it is cloudy with a high level of pollution on a particular day is 0.3. Find the probability that there will be a high level of pollution on a day when it is cloudy.

Give your answer to three significant figures.

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Given equal populations of Type X and Type Y bacteria, with mutation rates of 20% and 80% respectively, if a mutated bacterium is found, what's the probability it's Type Y?

Give your answer to three significant figures.

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Conditional Probability:

Conditional probability is a measure of the likelihood of an event occurring given that another event has already occurred. It represents the probability of event A happening, given that event B has already occurred. It is denoted as P(A|B), read as "the probability of A given B."

The formula for conditional probability is:

$$P(A|B) = \frac{{P(A \cap B)}}{{P(B)}}$$

Where:

$P(A|B)$ is the conditional probability of event A given event B,

$P(A \cap B)$ is the probability of both events A and B occurring,

$P(B)$ is the probability of event B occurring.

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Conditional Probability

Find the probability of one event happening given that another event has already happened.

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