Does 15a + 7b = 13? : GMAT Data Sufficiency (DS)
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# Does 15a + 7b = 13?

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Manager
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Does 15a + 7b = 13? [#permalink]

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12 Aug 2012, 12:31
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25% (medium)

Question Stats:

69% (01:18) correct 31% (00:48) wrong based on 45 sessions

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Does 15a + 7b = 13?

(1) b = 9

(2) 21b = 39 − 45a

why isnt the 1st statement sufficient?
[Reveal] Spoiler: OA
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Re: Does 15a + 7b = 13? [#permalink]

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12 Aug 2012, 12:35
Does 15a + 7b = 13?

(1) b = 9. Not sufficient since no info about a.

(2) 21b = 39 − 45a --> re-arrange: 45a+21b=39 --> reduce by 3: 15a+7b=13. Sufficient.

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Re: Does 15a + 7b = 13? [#permalink]

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12 Aug 2012, 18:07
Galiya wrote:
Does 15a + 7b = 13?

(1) b = 9

(2) 21b = 39 − 45a

why isnt the 1st statement sufficient?

b = 9 .. a = ?? what ? as long as you dont know the value of a you cant find the value of 15a + 21b
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Re: Does 15a + 7b = 13? [#permalink]

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12 Aug 2012, 21:49
Stmt 1 : if we put the value of b=9 in equation 15a+7b=13, we get the value of a=-10/3
mow again puting value of a=-10/3 and b= 9 in 15a+7b we get 13 as solution.

stmt 2 : sraight forward sufficient.

so, shouldnt the answer be D?
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Re: Does 15a + 7b = 13? [#permalink]

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12 Aug 2012, 22:07
bhavinshah5685 wrote:
Stmt 1 : if we put the value of b=9 in equation 15a+7b=13, we get the value of a=-10/3
mow again puting value of a=-10/3 and b= 9 in 15a+7b we get 13 as solution.

stmt 2 : sraight forward sufficient.

so, shouldnt the answer be D?

You don't have the equation $$15a + 7b = 13$$ because this is the question itself "Is $$15a + 7b = 13$$ ?"
You just have the value of $$b = 9$$ and you don't know anything about $$a$$, which can be anything.
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Re: Does 15a + 7b = 13? [#permalink]

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17 Aug 2012, 03:29
1
KUDOS
ST1: To get the value of 15a+7b
we should have both a and b
but here only b is given
hence INSUFFICIENT
ST2:21b=39-45a
=>45a+21b=39
=>15a+7b=13
hence SUFFICIENT
so correct ans is B
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Re: Does 15a + 7b = 13? [#permalink]

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17 Aug 2012, 07:11
I think Galiya was trying to solve for a.

If the question was:
Given 15a + 7b = 13, what is the value of a + b?
(A) would be sufficient.

However, we are looking for the result of 15a + 7b.
Statement 1 only guys us 15a + 63. Since we don't know the value of a, we cannot find the value of 15a + 63.
Statement 2 gives us an equivalent equation to 15a+7b: 45a+21b=39 -> 15a+7b=13
Since we know the value of 15a + 7b, statement 2 is sufficient.
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Re: Does 15a + 7b = 13? [#permalink]

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06 Dec 2012, 21:56
Statement (1) b = 9

15a + 7(9) = 13
15a + 63 = 13
15a = -50
a = -50/15

a could be anything. Thus, INSUFFICIENT!

Statement (2) 45a + 21b = 39

Divide the equation by 3 on both sides: 15a + 7b = 13 Yes! Thus, SUFFICIENT!

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Re: Does 15a + 7b = 13?   [#permalink] 06 Dec 2012, 21:56
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