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GMAT Prep?? (Arithmetic Sequence)

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by dferm » Thu Aug 28, 2008 7:05 am
Please Help....
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Source: — Data Sufficiency |
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by bomond » Thu Aug 28, 2008 10:22 am
ST 1. INSUFF.

We can see from the formula that sequence is arithmetic progression. Because it's increasing(decreasing) in the same constant number k

Then we can write a1, a2, ..., a15 consequence like this
a1, a1+k, a1+2k, ...,a1+14k

From the 1 ST. a1=24. Then consequence looks like this:
24, 24+k,24+2k, ...,24+14k.

While we don't know whether k is negative or positive we couldn't decide which term is greater than 10.
Let it be k=-2 Then we could count it.
Or let it be k=2 Then another number will be the answer. That's why INSUFF.

ST 2. We can write the consequence like this:
a8-7k, a8-6k, ..., a8-k, a8, a8+k, ..., a8+7k.

If k is positive number than last 7 terms of the consequence will be greater than 8. Because a8=10. a9,a10,... are greater than a8

If k is negative we can also surely say that first 7 terms are greater than 10.

So, in both cases we could surely count how many terms are greater than 10. It is 7. SUFF

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by rishi235 » Thu Aug 28, 2008 10:30 am
Very nice explanation...
Thank you bomond....
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by dferm » Thu Aug 28, 2008 10:53 am
I still don't understand the logic...am i missing something here...
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