6 DS answer explanation needed / GMATPrep Test 1

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kourousafama: Junior | Next Rank: 30 Posts; Posts: 21; Joined: Thu Jul 01, 2010 8:40 am; Location: London

by kourousafama » Thu Jul 29, 2010 9:18 am

Hi,

Could anyone help with the 6 attached DS questions answer explanation.
I have just past the GMATPrep Test 1 and would like to understand how I could have got these questions right.

Many thanks.

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Source: Beat The GMAT — Problem Solving | Thu Jul 29, 2010 9:18 am

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Thu Jul 29, 2010 8:00 pm

1st Question:

(1) 2x - 2y = 1 implies x - y = 1/2
Now, if x = 1 and y = 1/2, then x - y = 1/2. Here, x and y are both positive.
If x = 0 and y = -1/2, then x - y = 0 - (-1/2) = 1/2. Here, x and y are not positive.
We don't get a unique answer.
So, (1) is NOT SUFFICIENT.

(2) x/y > 1 implies that either x and y are both positive or x and y are both negative. But this is NOT SUFFICIENT to answer the question again.

Combining (1) and (2), if x = 1 and y = 1/2, then x - y = 1/2. Also x/y > 1 here. If we take negative values for both x and y, then x - y = 1/2 is not satisfied. Hence, x and y can never be negative.
So, answer to the main question is yes.

The correct answer is [spoiler](C)[/spoiler].

Rahul Lakhani
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https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
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GMAT/MBA Expert

Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Thu Jul 29, 2010 8:13 pm

2nd question:

(1) QR = RS implies angle RQS = angle RSQ = y, say.
But, (1) is NOT SUFFICIENT to find x.

(2) ST = TU implies angle TSU = angle SUT = z, say.
But, (2) is NOT SUFFICIENT to find x.

Combining (1) and (2), x + y + z = 180Âº
Also, angle PRT + angle PTR = 90Âº (because angle RPT = 90Âº)

Also, angle PRT = 180 - 2y and angle PTR = 180 - 2z
So, 180 - 2y + 180 - 2z = 90
or y + z = 135, implies x = 180 - 135 = 45Âº

The correct answer is [spoiler](C)[/spoiler].

Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)

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GMAT/MBA Expert

Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Thu Jul 29, 2010 8:32 pm

3rd question:

(1) If 8k = 16 then k = 2 and k is not divisible by 4.
If 8k = 32 then k = 4, divisible by 4.
We don't get a unique answer.
So, (1) is NOT SUFFICIENT.

(2) If 9k = 12 then k is not an integer.
If 9k = 24 then also k is not an integer.
If 9k = 36 then k = 4, divisible by 4
If 9k = 48 then also k is not an integer.
If 9k = 60 then also k is not an integer.
If 9k = 72 then k = 8, divisible by 4.

Generalizing, we can say k is divisible by 4 if 9k is divisible by 12.

The correct answer is [spoiler](B)[/spoiler].

Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)

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4GMAT_Mumbai: Master | Next Rank: 500 Posts; Posts: 161; Joined: Mon Apr 05, 2010 9:06 am; Location: Mumbai; Thanked: 37 times

by 4GMAT_Mumbai » Thu Jul 29, 2010 10:06 pm

4th Question:

Even before jumping into statements 1 and 2, let us look at the question.

Time taken if she had done x mph throughout = 40 / x

Time taken in the current scenario = (y/x) + ((40-y)/x)

Ratio of these two = ((y/x) + ((40-y)/x) ) / (40 / x)

It simplifies to (Y + ((40-Y)/1.25))

Stmt 1: Provides info about x. But, not useful as the expression has no x in it.

Stmt 2: Provides info about y. It will help to get a numeric value to the ratio. [spoiler]Hence, sufficient; B[/spoiler]

Hope this helps. Thanks.

Naveenan Ramachandran
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai

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4GMAT_Mumbai: Master | Next Rank: 500 Posts; Posts: 161; Joined: Mon Apr 05, 2010 9:06 am; Location: Mumbai; Thanked: 37 times

by 4GMAT_Mumbai » Thu Jul 29, 2010 10:10 pm

5th Question:

Stmt 1:
Let, Rows = C - 1 and Chairs = C

What is given?

C (C-1) = 72

C ^ 2 - C - 72 = 0. Solving this, we get C = 9. Hence, Sufficient.

Stmt 2:

Let Chairs = C

What is given?

C + (C-1) = 17.

C = 9. Hence, Sufficient.

Overall, D

Hope this helps. Thanks.

Naveenan Ramachandran
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai

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4GMAT_Mumbai: Master | Next Rank: 500 Posts; Posts: 161; Joined: Mon Apr 05, 2010 9:06 am; Location: Mumbai; Thanked: 37 times

by 4GMAT_Mumbai » Thu Jul 29, 2010 10:15 pm

6th Question:

Statement 1:

x = +1 or -1

But, the question states that x is -ve. Hence, x = 1. Suff

Statement 2:

Solving the equation leads to x = -2 or -1. Hence, Insufficient.

Confused ?!?!? Question you should ask yourself is "Does the statement lead me to be able to provide ONE single answer to the question?" The answer to this question is an YES in the first case and a NO in the second case. This is the reason why the first statement is sufficient and the second is not.

Hope this helps. Thanks.

Naveenan Ramachandran
4GMAT, Dadar(W) & Ghatkopar(W), Mumbai

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