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# 1-c-d=? 1) c+d=-1 2) c=-1-d

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Math Revolution GMAT Instructor
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1-c-d=? 1) c+d=-1 2) c=-1-d  [#permalink]

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07 Jul 2017, 01:06
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Difficulty:

15% (low)

Question Stats:

77% (00:49) correct 23% (00:44) wrong based on 57 sessions

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1-c-d=?

1) c+d=-1
2) c=-1-d

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Re: 1-c-d=? 1) c+d=-1 2) c=-1-d  [#permalink]

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07 Jul 2017, 02:54
MathRevolution wrote:
1-c-d=?

1) c+d=-1
2) c=-1-d

It is D.

From 1:

c + d = -1

multiplying both sides by -1

-c -d = 1

subtracting it from 1

1 - c - d = 1 - 1 = 0

From 2:

c = -1 - d

c + d = -1

Again we end up with the equation from 1 which can be used so answer is D.
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Re: 1-c-d=? 1) c+d=-1 2) c=-1-d  [#permalink]

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07 Jul 2017, 12:29
We have been asked to find the value of 1-c-d
This can be further simplified into 1-(c+d)

1) c+d=-1
Knowing the value of c+d, we can give a definite value for the expression. Sufficient

2) c=-1-d
Rewriting c+d = -1
Using same reason as above, Sufficient(Option D)
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Re: 1-c-d=? 1) c+d=-1 2) c=-1-d  [#permalink]

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07 Jul 2017, 19:36
Looking at the problem, 1-c-d can be changed to 1-(c+d)

Given the 1) and 2) are both forms of: c+d=-1, either can be used to solve the problem.

Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 7372
GMAT 1: 760 Q51 V42
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Re: 1-c-d=? 1) c+d=-1 2) c=-1-d  [#permalink]

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09 Jul 2017, 18:28
==> In the original condition, there are 2 variables (c,d) and in order to match the number of variables to the number of equations, there must be 2 equations. Since there is 1 for con 1) and 1 for con 2), C is most likely to be the answer. By solving con 1) and con 2), you get con 1) = con 2), so both of them becomes c+d=-1, and 1-c-d=1-(c+d)=1-(-1)=2, hence it is unique and sufficient.

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Re: 1-c-d=? 1) c+d=-1 2) c=-1-d   [#permalink] 09 Jul 2017, 18:28
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