Probability DS

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rahulvsd: Master | Next Rank: 500 Posts; Posts: 184; Joined: Tue Sep 07, 2010 9:43 amFollowed by: 1 member; Thanked: 6 times

by rahulvsd » Sat Oct 29, 2011 5:16 am

p is the probability of snow fall on any given day in the first week of December. If snow fall on one day does not affect the probability of snow fall on any other day, what is the value of p?

(1) The probability that snow will fall on at least one day during the first week of December is .918.

(2) The probability that snow will fall every day during this week is .00022

OA:D

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Source: Beat The GMAT — Data Sufficiency | Sat Oct 29, 2011 5:16 am

GmatMathPro: GMAT Instructor; Posts: 349; Joined: Wed Sep 28, 2011 3:38 pmFollowed by: 54 members; Location: Austin, TX; Thanked: 236 times; GMAT Score:770

by GmatMathPro » Sat Oct 29, 2011 6:34 am

Statement 1: If p is the probability of snow on any given day, the probability of it not snowing is 1-p. The probability of it snowing on at least one day during the week is 1-(the probability of it not snowing at all. The probability of it not snowing at all that week is (1-p)^7, so the probability of it snowing AT LEAST one day is 1-(1-p)^7, which we know is 0.918, so set 1-(1-p)^7=0.918 which you could clearly solve for p. SUFFICIENT.

Statement 2: The probability of snow falling every day in the first week is p^7. p^7=0.00022. Clearly this could be solved for p. SUFFICIENT.

Ans: D

Pete Ackley
GMAT Math Pro
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