Timed Quant Practice - Medium Section 2 Sample Test #2

GRE Quant Problem Solving
1

Hello. For this question I just wanted to confirm if my understanding is correct.

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The solution to this was to divide the positive possibilities by the total number of possibilities. My approach was to get the ratio of blue balls to total number of ball and the ratio of red balls to total number of balls -1 and multiply both the ratios. So I think I went wrong in understanding what the question asked. If the question asked what is the probability that the first ball that is picked is red and second ball is blue, my approach would have been right. But here since we are picking 2 balls in no specific order, we need to use combinatorics. Is this understanding correct?

2

Yes… Your thinking is right. Because there is no order of taking out, hence it can be Red First and Blue Second or Blue First and Red Second.
Hence in earlier approach if you add both possibilities, you will get right answer.

Blue = 2/6 = 1/3
Red = 3/5
Hence First Blue, Second Red = 2/6 * 3/5 = 1/5

Red = 3/6
Blue = 2/5
Hence First Red, Second Blue = 3/6 * 2/5 = 1/5

Adding them gives, 2/5.

3

Okay makes sense. So the only thing I missed was considering both possibilites. Thanks, this was helpful

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