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02 Nov 2019, 13:56
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Is it safe to assume as a thumb rule that if we have 3 variables and only 2 equations, we will not be able to get the answer.
Ref. OG question:
If x, y, and z are positive numbers, what is the value of the average (arithmetic mean) of x and z ?
1 )x − y = y − z
2) x2 − y2 = z
Even on combining the 2, you will not get the answer, because you need to find the value of one variable and you only have 2 equations.
Please confirm if this can be taken as a general rule? Can there be any exceptions to this?
Ref. OG question:
If x, y, and z are positive numbers, what is the value of the average (arithmetic mean) of x and z ?
1 )x − y = y − z
2) x2 − y2 = z
Even on combining the 2, you will not get the answer, because you need to find the value of one variable and you only have 2 equations.
Please confirm if this can be taken as a general rule? Can there be any exceptions to this?
02 Nov 2019, 20:44
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Tanichi
No, it cannot be a general rule.
Of course chances of E is much more but you cannot ignore C as the answer.
For example if statement ii was y=4, answer would be C.
Hope it helps
03 Nov 2019, 20:58
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Hi Tanichi,
Data Sufficiency questions have no 'safety net' - meaning that if you make a little mistake, then you won't realize it (you'll just convince yourself that one of the wrong answers is correct).
Your question is essentially about "system math" - the idea that if you want to find the exact value of all variables in a system, then you need that same number of UNIQUE equations. However, many DS prompts that appear to be 'system' questions are NOT actually asking you to solve a system (even though you might think that that is what you are looking at).
For example, if a DS question asks "What is the value of 2X - 3Y?".... then you might think to yourself "I need the value of X and I need the value of Y, so I need two unique equations." While that is ONE way to get to the answer, that's NOT the only way. Consider the following:
1) 4X - 6Y = 14
This is just one equation.... but you can get the answer to the question IF you divide this equation by 2...
4X - 6Y = 14
2X - 3Y = 7
We now have the answer to the question that was asked... and we needed just one equation. Fact 1 is SUFFICIENT on its own and we don't need a 2nd equation.
This is meant to say that if you think that you are dealing with an INSUFFICIENCY in a DS question, then you should be able to prove it. Your post implies that you're looking for a reason to not do work.... and when you choose to not do any work (and assume that a math pattern exists), then you're either correct OR you're about to make a little mistake and let some easy points get away.
GMAT assassins aren't born, they're made,
Rich
Data Sufficiency questions have no 'safety net' - meaning that if you make a little mistake, then you won't realize it (you'll just convince yourself that one of the wrong answers is correct).
Your question is essentially about "system math" - the idea that if you want to find the exact value of all variables in a system, then you need that same number of UNIQUE equations. However, many DS prompts that appear to be 'system' questions are NOT actually asking you to solve a system (even though you might think that that is what you are looking at).
For example, if a DS question asks "What is the value of 2X - 3Y?".... then you might think to yourself "I need the value of X and I need the value of Y, so I need two unique equations." While that is ONE way to get to the answer, that's NOT the only way. Consider the following:
1) 4X - 6Y = 14
This is just one equation.... but you can get the answer to the question IF you divide this equation by 2...
4X - 6Y = 14
2X - 3Y = 7
We now have the answer to the question that was asked... and we needed just one equation. Fact 1 is SUFFICIENT on its own and we don't need a 2nd equation.
This is meant to say that if you think that you are dealing with an INSUFFICIENCY in a DS question, then you should be able to prove it. Your post implies that you're looking for a reason to not do work.... and when you choose to not do any work (and assume that a math pattern exists), then you're either correct OR you're about to make a little mistake and let some easy points get away.
GMAT assassins aren't born, they're made,
Rich
