# Represent and Determine Probability Question

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## Grade 10 Represent and Determine Probability CCSS: HSS-CP.B.6

There are 150 grade 12 students at East York Collegiate Institute.

70 of the grade 12 students drive a car at the school.

Half of all students with a grade average of 90 or higher have a car.

She is asked to find the probability of randomly choosing a grade 12 student with a car, given that the student has an average grade of 90 or higher. She reasons initially that, since there are 70 grade 12 students out of 150 who have a car, it would be 7/15. However, since it is a conditional probability, she needs to multiply by 1/2, since the probability that a student with a grade average of 90 or higher is 1/2. This results in a probability of 7/30. Is she correct or not, and why?

- Yes, and her reasoning is correct.
- No, she didn't need to multiply by 1/2 and the probability is 7/15. The probability that a student has a car can be assumed to be independent of their grade average, and so no multiplication is needed.
- No, there is insufficient information to find this probability.
- No, she needs to state that it can be assumed that half of grade 12 students have an average of 90 or higher. Therefore, half of 70 is 35, half of 150 is 75, and the probability is 7/15.