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Probability. In how many seconds?

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by kamu » Wed Mar 04, 2009 1:58 pm
The cards in a deck are numbered consecutively from 1 to 20. If some cards are red and some
cards are black, what is the probability of drawing a red card?
(1) The odd-numbered cards are all red.
(2) The even-numbered cards are all black.

How much time did you take?

OA : C
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by Feep » Wed Mar 04, 2009 4:34 pm
In order to know the probability of such an event, we need to know how many cards are red and how many are black, without ambiguity.

(1) tells you about ten cards, but obviously there are ten we know nothing about.

(2) also tells you about ten cards, but once again, there are ten we know nothing about.

(1) and (2) together reveal the necessary information about all twenty cards, and so we may determine the probability (1/2).

This isn't a very difficult question, but as long as you nail it within a reasonable amount of time, you should feel comfortable. Don't feel as though you need to get every "easy" question in thirty seconds, or something.
I tutor GMAT/GRE level mathematics privately in the Los Angeles region, as well as via Skype for a discounted rate. Send me a message if you're interested.
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by kamu » Thu Mar 05, 2009 7:25 am
When I first read it I thought I'd it in 10s.

Then when I thought about it I was incorrect.

May be I was sleepy or something. :)
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