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# GMATprep DS: Number theory II

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VP
Joined: 21 Mar 2006
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GMATprep DS: Number theory II [#permalink]  09 Feb 2007, 02:25
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
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Director
Affiliations: FRM Charter holder
Joined: 02 Dec 2006
Posts: 736
Schools: Stanford, Chicago Booth, Babson College
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Kudos [?]: 38 [0], given: 4

In any case there can 7 numbers that meet the criterion.
Manager
Joined: 14 Apr 2003
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Tricky statement is 2<= n <=15

For stmnt 1.
========
a(1) = 24
If K is +ve then all are more than 10. If K is -24 all can be -ve so CANNOT decide.

For stmnt 2.
========
If a(8) = 10 then if K is +ve then all the elements for n > 8 are > 10.

If K -ve all the elements for n < 8 are > 10.

I would have chosen B actually I did. Then I reliased that n can only go up to 2.

In that case I would go with C.

OA please on this. I need to check whether this has a trap or not.

vineet
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Re: GMATprep DS: Number theory II [#permalink]  09 Feb 2007, 10:00
kripalkavi wrote:

Coniditon1: a1 = 24

a2 = a1 + k = 24 + k
a3 = a2 + k = 24 + 2k

a15 = 24 + 14k

we can keep on doing this but we will not be able to find all the values as we don't know the value of k it can be negative or positive. So it is hard to say how many values are greater
than 10.

a8 = a7 + k = 10 ==> a7 = 10-k
a9 = a8 + k = 10 + k

a7 = 10-k
a8 = 10
a9 = 10+k

if k is psotive we know all the value greater than a8 will be greater than 10
if k is negative we know all the values less than a8 will be greater than 10

and that number will be 7 terms so condition 2 is sufficient answer is B
_________________

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VP
Joined: 21 Mar 2006
Posts: 1135
Location: Bangalore
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Kudos [?]: 29 [0], given: 0

Re: GMATprep DS: Number theory II [#permalink]  10 Feb 2007, 03:36
amorpheus wrote:
kripalkavi wrote:

Coniditon1: a1 = 24

a2 = a1 + k = 24 + k
a3 = a2 + k = 24 + 2k

a15 = 24 + 14k

we can keep on doing this but we will not be able to find all the values as we don't know the value of k it can be negative or positive. So it is hard to say how many values are greater
than 10.

a8 = a7 + k = 10 ==> a7 = 10-k
a9 = a8 + k = 10 + k

a7 = 10-k
a8 = 10
a9 = 10+k

if k is psotive we know all the value greater than a8 will be greater than 10
if k is negative we know all the values less than a8 will be greater than 10

and that number will be 7 terms so condition 2 is sufficient answer is B

Awesome explanation! OA is B.
Re: GMATprep DS: Number theory II   [#permalink] 10 Feb 2007, 03:36
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