probability D.S

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raunakrajan: Senior | Next Rank: 100 Posts; Posts: 62; Joined: Tue Mar 16, 2010 8:27 am; Thanked: 1 times

by raunakrajan » Wed Jun 30, 2010 3:44 am

If a number n is pulled out of a hat at random, what is the probability that( 1/2)n - 2 â‰¥ 0?

(1) There are 6 numbers in the hat and each number is an integer.

(2) For every value of n in the hat, 1 â‰¤ n â‰¤ 30.

ANSWER IS E
but how to go about this?

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Source: Beat The GMAT — Data Sufficiency | Wed Jun 30, 2010 3:44 am

Rich@VeritasPrep: GMAT Instructor; Posts: 147; Joined: Tue Aug 25, 2009 7:57 pmFollowed by: 17 members; Location: New York City; Thanked: 76 times; GMAT Score:770

by Rich@VeritasPrep » Wed Jun 30, 2010 4:02 am

First, you can rewrite the inequality in the prompt:

(1/2)n - 2 >= 0

(1/2) n >= 2

n >= 4

So the question really asks for the probability that n >= 4.

(1) doesn't tell you anything, because we don't know what integers are in the hat.

(2) also doesn't tell you anything, because although we know every number in the hat is between 1 and 30 inclusive, we don't know how many are equal to 4 or greater.

(1) and (2) together still don't tell you anything. We know there are 6 integers in the hat, and we know that all of the integers are between 1 and 30 inclusive, but there's no way of telling how many of the integers are greater than or equal to 4.

Rich Zwelling
GMAT Instructor, Veritas Prep

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