WORD TRANSLATION 4th EDITION, CHPTR 5 PG 93 Q9

[kadeneclarke]

How do you know when to assume 'without replacement' if it's not stated in the question. For e.g. q9 states "In a bag of marbles, there are 3 red, 2 white and 5 blue. If Bob takes 2 marbles, what's the probability that he will have one white and one blue marble?". How would I know that Bob's marble selection took place without replacement?

Thanks!

[jnelson0612]

How do you know when to assume 'without replacement' if it's not stated in the question. For e.g. q9 states "In a bag of marbles, there are 3 red, 2 white and 5 blue. If Bob takes 2 marbles, what's the probability that he will have one white and one blue marble?". How would I know that Bob's marble selection took place without replacement?

Thanks!

Hi Kaden,
Replacement is only an issue if you are choosing objects one at a time. For example, if I pick two marbles one at a time, I have two scenarios:
1) The first marble chosen has been replaced and I have the same number of marbles to choose from both times.
2) I have one fewer marble to choose from on the second draw since the first marble was not replaced.

These two scenarios will give us different probabilities for the second draw.

Contrast this to choosing two marbles at once: you just reach in and pull out two marbles simultaneously. There's no issue of replacement since there isn't a later draw.

I hope this makes sense!

[kadeneclarke]

Jamie,
Your explanation does make sense but note the question didn't state whether the marbles were selected one at a time or all at once. Why was the former scenario chosen to solve the question?

[jnelson0612]

Good question kaden. In this type of question, you can *generally* assume that the marbles are drawn one at a time. Frankly, on a real GMAT problem the test writers will make sure there is no ambiguity. :-)