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12 Jul 2011, 12:18
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Hello everyone,
Assume we have this DS question.
If y and n are positive integers, is yn divisible by 7?
(1) n^2 - 14n + 49=0
(2) n + 2 is the first of three consecutive integers whose product is 990.
Now, from Statement (1) I can derive that n=7 which means that (1) is sufficient.
For Statement (2), can I use the value from (1) and plug it in (n + 2) * (n + 3) * (n + 4) which gives me 9*10*11=990?
To make my question clearer, can I use values from either statement (if I am 100% sure that a certain statement is sufficient) to evaluate the other statement (that I am not sure about)?
The answer to this question is
(D): EACH statement ALONE is sufficient.
Thank you
Assume we have this DS question.
If y and n are positive integers, is yn divisible by 7?
(1) n^2 - 14n + 49=0
(2) n + 2 is the first of three consecutive integers whose product is 990.
Now, from Statement (1) I can derive that n=7 which means that (1) is sufficient.
For Statement (2), can I use the value from (1) and plug it in (n + 2) * (n + 3) * (n + 4) which gives me 9*10*11=990?
To make my question clearer, can I use values from either statement (if I am 100% sure that a certain statement is sufficient) to evaluate the other statement (that I am not sure about)?
The answer to this question is
(D): EACH statement ALONE is sufficient.
Thank you
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12 Jul 2011, 16:13
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The Statements in any real Data Sufficiency question are always consistent. So if you find that n=7 is the only possible value of n from Statement 1, then 100% of the time the value n=7 will 'work' with Statement 2. Unfortunately, that usually doesn't help you very much. There will always be at least one solution for n using either Statement. In Data Sufficiency questions, your task is to determine if there is *only* one solution, or if there is more than one solution. So yes, using the value you find from Statement 1, you find one solution immediately for Statement 2, but that doesn't answer the question - you still need to work out if there might be other solutions.
To give a simpler example than the question in your post, if you have the question:
x and y are consecutive integers, and x < y. What is the value of x?
1) x + y = 7
2) xy = 12
You can work out that x = 3 using Statement 1. Of course that solution works with Statement 2 as well, but that doesn't mean very much. Statement 2 is not sufficient, since x could be 3 or could be -4.
To give a simpler example than the question in your post, if you have the question:
x and y are consecutive integers, and x < y. What is the value of x?
1) x + y = 7
2) xy = 12
You can work out that x = 3 using Statement 1. Of course that solution works with Statement 2 as well, but that doesn't mean very much. Statement 2 is not sufficient, since x could be 3 or could be -4.
12 Jul 2011, 16:28
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I think Ian's reply is a good one. Although, I do think in some circumstances using an answer from 1 can be a useful trick in your armory (although definitely not a primary tool)
For instance in your example, say you look at
2) n+2 is the first of three consecutive integers whose product is 990.
My thought process for this is:
- looks like three close numbers = ~1000, i.e. n^3 = 1000, n = 10
- consecutive integers could be 9,10,11...
- 9*10*11 = 990, therefore n = 7.
- yn is therefore divisible, this is sufficient.
But lets say that you didn't go through that thought process and stared at 2 for a little too long. Then i think you are right that you could use n = 7 from statement 1 to get to
- 9*10*11 = 990 and follow thought process from there
At this stage like Ian said you would want to gain comfort that this is in fact the only solution, and that 2 can be used on its own. So you would have to make sure you put 1) out of mind and assess 2) in its own right post using 1) as a prompt.
For instance in your example, say you look at
2) n+2 is the first of three consecutive integers whose product is 990.
My thought process for this is:
- looks like three close numbers = ~1000, i.e. n^3 = 1000, n = 10
- consecutive integers could be 9,10,11...
- 9*10*11 = 990, therefore n = 7.
- yn is therefore divisible, this is sufficient.
But lets say that you didn't go through that thought process and stared at 2 for a little too long. Then i think you are right that you could use n = 7 from statement 1 to get to
- 9*10*11 = 990 and follow thought process from there
At this stage like Ian said you would want to gain comfort that this is in fact the only solution, and that 2 can be used on its own. So you would have to make sure you put 1) out of mind and assess 2) in its own right post using 1) as a prompt.
