Probability. In how many seconds?

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kamu: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Fri Feb 27, 2009 12:41 am; Thanked: 44 times; Followed by:3 members

Probability. In how many seconds?

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

Quote

Source: Beat The GMAT — Data Sufficiency | Wed Mar 04, 2009 1:58 pm

Feep: Senior | Next Rank: 100 Posts; Posts: 69; Joined: Wed Mar 04, 2009 4:15 pm; Thanked: 17 times; GMAT Score:780

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.

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