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31 Jan 2016, 07:37
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A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
35%
(medium)
Question Stats:
71% (01:31) correct
29%
(02:08)
wrong
based on 70
sessions
History
Date
Time
Result
Not Attempted Yet
31 Jan 2016, 07:53
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Bunuel
Hi,
A good Q again..
what are we required to find..
Is the product cd positive?
what it means is- are both c and d of same sign?
lets check the statements..
(1) 3c = −8d^3
THREE CASES:-
A)If c is positive, -8d^3 should be positive.. this will make d as negative..
B)If c is negative, -8d^3 should be negative.. this will make d aspositive..
C) we should not miss out on this case where c=d=0..
From A and B, we know that c and d are of different sign, so cd will be negative.. answer to Is the product cd positive? is NO..
From C, cd=0, again answer to Is the product cd positive? is NO..
since the answer in all circumstances is NO, it is SUFFICIENT
(2) d > c + 4
We cannot say anything about signs ..
insuff
ans A
01 Feb 2016, 05:43
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Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.
Is the product cd positive?
(1) 3c = −8d^3
(2) d > c + 4
When you modify the original condition and the question, they becomes
cd>0?
1) cd<=0
2) d>c+4
For 1), it is no and sufficient. Therefore, the answer is A.
That is, in the original condition, there are 2 variables(c,d), which should match with the number of equations. So you need 2 equation. For 1) 1 equation, for 2) 1 equation, which is likely to make C the answer. Even for inequality question, one of the key questions, you have to be careful of the mistake type 4(A).
According to 1), it is sufficient. Therefore, the answer is A>
-> For cases where we need 2 more equations, such as original conditions with “2 variables”, or“3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
Is the product cd positive?
(1) 3c = −8d^3
(2) d > c + 4
When you modify the original condition and the question, they becomes
cd>0?
1) cd<=0
2) d>c+4
For 1), it is no and sufficient. Therefore, the answer is A.
That is, in the original condition, there are 2 variables(c,d), which should match with the number of equations. So you need 2 equation. For 1) 1 equation, for 2) 1 equation, which is likely to make C the answer. Even for inequality question, one of the key questions, you have to be careful of the mistake type 4(A).
According to 1), it is sufficient. Therefore, the answer is A>
-> For cases where we need 2 more equations, such as original conditions with “2 variables”, or“3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
