Probability

Practice sheet

Fresh questions from the whole chapter, to do on paper.

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  1. Two fair dice are rolled. Find the probability that the sum is 2.
  2. A fair die is rolled 2 times. Find the probability of at least one six.
  3. The probability that a flight is late is 0.78. Find the probability that it is not late.
  4. In a class of 35 students, 12 are girls, 12 play soccer and 2 girls play soccer. A student is chosen at random. Find P(girl or soccer player).
  5. Events A and B are mutually exclusive, with P(A) = 0.31 and P(B) = 0.24. Find P(A and B).
  6. A spinner lands on red with probability 0.21 and on blue with probability 0.32. Find P(red or blue).
  7. A screening test was given to 316 people: 47 had the disease and tested positive, 8 had the disease and tested negative, 34 did not have it and tested positive, 227 did not have it and tested negative. Find P(disease | positive).
  8. P(A) = 0.60, P(B) = 0.22 and P(A and B) = 0.5. Find P(A | B).
  9. A test detects a disease in 87% of people who have it. Which statement says the same thing?
    1. P(positive | disease) = 0.87
    2. P(disease | positive) = 0.87
    3. P(disease and positive) = 0.87
  10. Two independent machines fail with probabilities 0.8 and 0.7. Find the probability that both fail.
  11. P(A) = 3/10, P(B) = 6/10 and P(A and B) = 18/100. Are A and B independent?
    1. Yes: P(A and B) = P(A)·P(B)
    2. No, because P(A and B) > 0
    3. It cannot be decided
  12. A bag has 5 red and 3 blue marbles. Two are drawn without replacement. Find P(both red).