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Source: — Data Sufficiency |
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by theCodeToGMAT » Wed Oct 16, 2013 12:20 am
Terms in "S" = 240
To find: 239th

Statement 1:
Difference between numbers = 4.. Series is AP
But we have no idea of first or any other term
INSUFFICIENT

Statement 2:
239th = First - 952
We dont know whether it's AP or not.
Also, we dont have specific values
INSUFFICIENT

Combining...
Series is AP
239th - 1st = 952
We don't know the value of any term .. so we cannot conclude the values of any term.
INSUFFICIENT

Answer [spoiler]{E}[/spoiler]
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by rakeshd347 » Wed Oct 16, 2013 12:31 am
theCodeToGMAT wrote:Terms in "S" = 240
To find: 239th

Statement 1:
Difference between numbers = 4.. Series is AP
But we have no idea of first or any other term
INSUFFICIENT

Statement 2:
239th = First - 952
We dont know whether it's AP or not.
Also, we dont have specific values
INSUFFICIENT

Combining...
Series is AP
239th - 1st = 952
We don't know the value of any term .. so we cannot conclude the values of any term.
INSUFFICIENT

Answer [spoiler]{E}[/spoiler]
Hi Rahul,

I chose C and got it wrong. Please let me know where is my reasoning wrong. If possible by some picking number. I thought as the difference between 1st and 239th term is 952 and the difference between each term is 4 then if you divide 952 it turns out to be 238 so 1st term would have been 0 obviously and the 240th term will be 956. So where did I go wrong?
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by theCodeToGMAT » Wed Oct 16, 2013 1:03 am
Let the first number be "x"

239th Term = x - 952

Difference = 4

So series
x , x-4 , x-8, x-19 ...... x-952, x-956

If X = 0 , 239th Term = -952
X = 1000, 239th Term = 48

So, we are not sure.
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by lunarpower » Wed Oct 16, 2013 2:14 am
It seems that, by simply typing out the question, you'd have spent only a fraction of the time needed to capture and upload an image. Hmm.
rakeshd347 wrote:I chose C and got it wrong. Please let me know where is my reasoning wrong. If possible by some picking number.
First, note that statement 2 doesn't add any value to statement 1. If you know that each term decreases by 4 from the previous term, then statement 2 is already a consequence!

In terms of picking numbers --
If the first term is 1000, then the 239th term is 1000 - 952.
If the first term is 10,000, then the 239th term is 10,000 - 952.

You don't have any specific terms of the sequence, so the 239th term can actually be any number in the entire world.
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by lunarpower » Wed Oct 16, 2013 2:15 am
I thought as the difference between 1st and 239th term is 952 and the difference between each term is 4 then if you divide 952 it turns out to be 238
This 238 is just the number of terms between those two values. (Not new information; you already know that they are the 1st and 239th terms, so they are 238 terms apart.)
It's not the value of anything.

so 1st term would have been 0 obviously
The use of the word "obviously" here is ... interesting, considering that this is the part of your reasoning that's incorrect.

(It's best to stay away from words like "easy", "obvious", and so on. There's no possible advantage of using those words, ever -- and, don't forget, people post these problems on the forum because they're not "obvious"!)
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by mevicks » Wed Oct 16, 2013 2:16 am
rakeshd347 wrote:If sequence S has 240 terms, what is the 239th term of S ?
(1) Each term of S after the first term is 4 less than the preceding term.
(2) The 239th term of S is 952 less than the first term.
We need the 239th element of the sequence. So if we know the relationship among the elements of the sequence AND a single value we can answer the question sufficiently.

Given: No of elements n = 240
Q: What is the 239th element i.e the last but one term?

St1:
sequence becomes : a, (a - 4), (a - 8), ... [a - (n - 1)4]

1st Term = a
2nd Term = a - (2 - 1)4
...
239th Term = a - (238)4 = [a - 952]
240th Term = a - (239)4

We require a single value or the value of a to find the last but one element, so INSUFFICIENT

St2:
239th element is 952 less than the first -->
a, ... [a - 952]

We require a single value or the value of a to find the last but one element, so INSUFFICIENT

St1+St2:
Combining the two statements essentially yield the same sequence:
a, [a - (1)4], [a - (2)4], ... [a - 952]
Here the last but one (239th) element is again [a - 952]
Even after combining we are left with the same equation and again we cannot find the last but one element.
INSUFFICIENT

[spoiler]Answer : E[/spoiler]

Regards,
Vivek
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