(I'm Overwhelmed) Data Analysis Progress Quiz #11, number 8

GRE GRE Quant Problem Solving
1

Hi! I’m just confused here, why in this case, we divided the target probability (651/671) with all probabilities allowed (671/1296)? Since usually what was taught to us, and even how previous questions were answered was to use 1296 as the denominator and just add the numerators of the target probability. In this case we divided the 2 fractions.

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How should I know when and when not to do this?

2

We need to consider favourable cases probability to total cases probability. As it is a given probability question, something needs to adjusted in the total cases.