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70% of houses in a residential area have an alarm. It is found that 35% of the owners of thealarmed houses have a dog and 42% of the owners of the non-alarmed houses have a dog.a) Construct a tree diagram to show all possible events and probabilities.b) A house is selected at random. Calculate the probability that the house does nothave a dog.c) If a randomly selected house is one of the houses that does not have a dog, what isthe probability that the house has an alarm?

Question

70% of houses in a residential area have an alarm. It is found that 35% of the owners of thealarmed houses have a dog and 42% of the owners of the non-alarmed houses have a dog.a) Construct a tree diagram to show all possible events and probabilities.b) A house is selected at random. Calculate the probability that the house does nothave a dog.c) If a randomly selected house is one of the houses that does not have a dog, what isthe probability that the house has an alarm?

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Solution

a) Tree Diagram:

  • Start with two branches. Label one as "Alarmed Houses (70%)" and the other as "Non-Alarmed Houses (30%)".
  • From the "Alarmed Houses" branch, draw two more branches. Label one as "With Dog (35%)" and the other as "Without Dog (65%)".
  • From the "Non-Alarmed Houses" branch, draw two more branches. Label one as "With Dog (42%)" and the other as "Without Dog (58%)".

b) Probability that the house does not have a dog:

  • For alarmed houses: 0.70 (probability of an alarmed house) * 0.65 (probability of not having a dog given the house is alarmed) = 0.455
  • For non-alarmed houses: 0.30 (probability of a non-alarmed house) * 0.58 (probability of not having a dog given the house is non-alarmed) = 0.174
  • Add these two probabilities: 0.455 + 0.174 = 0.629 or 62.9%

c) Probability that the house has an alarm given it does not have a dog:

  • We already calculated the probability of a house not having a dog as 0.629.
  • The probability of a house being alarmed and not having a dog is 0.455.
  • So, the conditional probability of the house having an alarm given it does not have a dog is 0.455 / 0.629 = 0.723 or 72.3%

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