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Pre-cal

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What is the probability of getting an even number on a single roll of a number cube if you roll a 3 or greater.

My answer: is 1/3 because there are only two numbers that are even and greater than 3 on the number cube.

Teacher marked this as wrong. How did I mess this one up?

  • Pre-cal -

    this sure looks like a "conditional" probability.
    in general
    P(A│B) = P(A AND B)/P(B)
    read: P(A│B) as "the prob of A given B"

    so let A be (even number)
    let B be (greater or equal to 3)

    so P(A and B) = 2/6 = 1/3
    P(B) = 4/6 = 2/3

    so P(A│B) = (1/3)/(2/3) = 1/2

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