A number is to be selected at random....

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The Jock: Master | Next Rank: 500 Posts; Posts: 207; Joined: Mon Oct 06, 2008 12:22 am; Location: India; Thanked: 5 times; Followed by:3 members

A number is to be selected at random....

by The Jock » Sun Aug 01, 2010 5:09 am

{-10,-6,-5,-4,-2.5,-1,0,2.5,4,6,7,10}
A number is to be selected at the random from the set above.what is the probability that the number selected will be a solution of the equation (x+5)(x+10)(2x-5) = 0?
1. 1/12
2. 1/6
3. 1/4
4. 1/3
5. 1/2

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Source: Beat The GMAT — Problem Solving | Sun Aug 01, 2010 5:09 am

this_time_i_will: Master | Next Rank: 500 Posts; Posts: 268; Joined: Wed Mar 17, 2010 2:32 am; Thanked: 17 times

by this_time_i_will » Sun Aug 01, 2010 5:15 am

the given polynomial would yield 0 for x = -5,-10,2.5
so required prob = 3/12 = 1/4

IMO C

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indiantiger: Master | Next Rank: 500 Posts; Posts: 362; Joined: Fri Oct 02, 2009 4:18 am; Thanked: 26 times; Followed by:1 members

by indiantiger » Sun Aug 01, 2010 12:54 pm

total elements in the set = 12
total solution of the given equation (x+5)(x+10)(2x-5) = 0
-5,-10,2.5

all these exist in the set

probability = relevant cases/total cases = 3/12 = 1/4 (option 3)

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jlazaridis: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Wed Sep 14, 2011 2:49 pm

by jlazaridis » Wed Sep 14, 2011 2:50 pm

Guys, this is wrong. The solutions are: +5, -10 and +2.5. However, from the list we only see 2 numbers: -10 and +2.5, therefore the probability is 2/12 which is equal to 1/6 (B)

indiantiger wrote:total elements in the set = 12
total solution of the given equation (x+5)(x+10)(2x-5) = 0
-5,-10,2.5

all these exist in the set

probability = relevant cases/total cases = 3/12 = 1/4 (option 3)

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pemdas: Legendary Member; Posts: 1084; Joined: Fri Apr 15, 2011 2:33 pm; Thanked: 158 times; Followed by:21 members

by pemdas » Wed Sep 14, 2011 3:11 pm

either one should be set equal to 0
x=-5, x=-10, x=2.5
three possibilities out of twelve => 1/4
c

this q. lacks symmetry in set, +1 is missing

The Jock wrote:{-10,-6,-5,-4,-2.5,-1,0,2.5,4,6,7,10}
A number is to be selected at the random from the set above.what is the probability that the number selected will be a solution of the equation (x+5)(x+10)(2x-5) = 0?
1. 1/12
2. 1/6
3. 1/4
4. 1/3
5. 1/2

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sl750: Master | Next Rank: 500 Posts; Posts: 496; Joined: Tue Jun 07, 2011 5:34 am; Thanked: 38 times; Followed by:1 members

by sl750 » Thu Sep 15, 2011 10:32 am

Solutions to the equation are -5,-10,5/2

We have these solutions in the set, which has 12 elements.

Probability of picking any of the 3 solutions out of 12 is 3/12 = 1/4

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mdkragh: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Wed Nov 16, 2011 4:00 pm

by mdkragh » Thu Jan 12, 2012 11:42 am

He copied the equation wrong from the sample test. It should say

{-10,-6,-5,-4,-2.5,-1,0,2.5,4,6,7,10}
A number is to be selected at the random from the set above.what is the probability that the number selected will be a solution of the equation (x-5)(x+10)(2x-5) = 0?
1. 1/12
2. 1/6
3. 1/4
4. 1/3
5. 1/2

That is why the answer is B 1/6 and not C 1/4.