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Data Sufficiency Question : Average

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by kfat » Mon Jul 12, 2010 3:58 pm
The average height of a group of children is 125 cm. If one of the children leaves, the average height drops by 2 cm. how many kids were there originally?
(1) The height of the child who left is twice greater than the height of the shortest child.
(2) The height of the child who left is 130 cm.
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by boazkhan » Mon Jul 12, 2010 4:08 pm
IMO A. what is the OA?
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by barcebal » Mon Jul 12, 2010 8:05 pm
I want to say B but something isn't right.

n=number of original kids

(123*(n-1) + 130)/n = 125 right?

Simplified you get 125n = 123n -123 + 130

Simplify again and you 2n = 7

N=7/2

You can't have 7/2 kids. We get a value but we don't get a real answer? Are you sure the numbers are right?

Statement I seems not sufficient too. You can't really use the info about the smallest kid to infer how many kids there are and write an equation like I did in Statement B because we don't know how the smallest kid affects the avg.
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by gmatmachoman » Mon Jul 12, 2010 11:40 pm
kfat wrote:The average height of a group of children is 125 cm. If one of the children leaves, the average height drops by 2 cm. how many kids were there originally?
(1) The height of the child who left is twice greater than the height of the shortest child.
(2) The height of the child who left is 130 cm.
St 2 should be sufficient. But the numbers are contradictory...
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by kfat » Tue Jul 13, 2010 7:26 am
Guys... The questions and figures in the questions both are correct. This problem was in 700-800 toughness level. Many a times persons sees a linear equation and marks the answer.

The answer for this question is "E". Person needs to solve to get n = 7/2 to avoid marking answer as "B".
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by GMATGuruNY » Tue Jul 13, 2010 8:55 am
kfat wrote:Guys... The questions and figures in the questions both are correct. This problem was in 700-800 toughness level. Many a times persons sees a linear equation and marks the answer.

The answer for this question is "E". Person needs to solve to get n = 7/2 to avoid marking answer as "B".
This question would never appear on the GMAT. The GMAT writers will never suggest an equation that results in 7/2 of a person.

(The justification for answer choice E seems to be that when you use the information given, you get 7/2 of a person, and since this number makes no sense, the statement is insufficient. You'll never face this situation on the GMAT.)
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by barcebal » Tue Jul 13, 2010 9:14 am
kfat wrote:Guys... The questions and figures in the questions both are correct. This problem was in 700-800 toughness level. Many a times persons sees a linear equation and marks the answer.

The answer for this question is "E". Person needs to solve to get n = 7/2 to avoid marking answer as "B".
I agree with GMATguruNY,

The information in the question is inherently true and so is each statement. So if the facts given in the question are TRUE and statement 2 is TRUE (I'm not saying sufficient, I'm saying TRUE) then you get n = 7/2, which cannot answer the question, NOT BECAUSE statement 2 is unsufficient, but because either (A) the question's facts are incorrect or (B) statement 2 is false, both of which CAN NOT occur because we're not being tested on whether the statement is true, but rather sufficient.
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