You are expecting two sets of exams- “easy” and “hard”. The probabilit

ikavumacj

ikavumacj

Answered question

2021-11-15

You are expecting two sets of exams- “easy” and “hard”. The probability of getting a hard exam is 0.80. The probability that the first question on the exam will be difficult is 0.90 if the set is “hard” and is 0.15 if the set is “easy.
a) What is the probability that the first question on your exam is marked as difficult.
b) What is the probability that your exam is hard given that the first question on the exam is marked as difficult?

Answer & Explanation

Lauren Fuller

Lauren Fuller

Beginner2021-11-16Added 14 answers

Step 1
It is given that the probability of getting a hard exam is 0.80.
Since there are only two sets of exams - "easy" and "hard".
Then the probability of getting an easy exam is 10.80=0.20.
Also given that, if the set is "hard" then the probability that the first question in the exam will be difficult is 0.90 and if the set is "easy" then the probability that the first question in the exam will be difficult is 0.15.
Step 2
a) Here we have to find the probability that the first question in the exam is marked as difficult.
Therefore, Required Probability is given by
P=P(getting easy set of exam)
P(that the first question in the easy set of exam will be difficult)+P(getting hard set of exam
P(that the first question in the hard set of exam will be difficult)
P=0.20×0.15+0.80×0.90
P=0.03+0.72
P=0.75
P=34
Thus, the required probability that the first question on your exam is marked as difficult is equal to 34 or 0.75.
Step 3
b) Here we have to find the probability that the exam is hard given that the first question on the exam is marked as difficult.
Therefore, Required Probability is given by
P=P(getting hard set of exam)P(that the first question in the hard set of exam will be difficult)P(that the first question in the exam is marked as difficult)
P=0.80×0.900.75
P=0.720.75
P=2425
Thus, the required probability that the exam is hard given that the first question on the exam is marked as difficult is equal to 2425 or 0.96.

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