Question

One of two small classrooms is chosen at random with equally likely probability, and then a student is chosen at random from the chosen classroom. Classroom #1 has 5 boys and 11 girls. Classroom #2 has 15 boys and 9 girls. What is the probability that Classroom #2 was chosen at random, given that a girl was chosen? Your answers should be rounded to 4 digits after the decimal.

Answer 1

0.1875

Explanation:

Let A be the event that class two has been chosen . So the probability of A would be P (A) = 1/2= 0.5

Now class two has 9 girls out of total 24 students . So the probability of chosing the girl would be= P (B=) 9/24= 0.375

So the probability that Classroom #2 was chosen at random, given that a girl was chosen is given by = P(A) . P(B)= 0.5 * 0.375= 0.1875

Another way of finding the probability that Classroom #2 was chosen at random, given that a girl was chosen is by drawing a tree diagram.

P (1/2) Class 1 ------------------------5 boys

-------------------------11 girls (11/16)

P (1/2) Class 2 -------------------------15 boys

-----------------------9 girls P (9/24) Class 2 was chosen (0.5) *9/24