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An integer greater than 1 that is not prime is called

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An integer greater than 1 that is not prime is called [#permalink]

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10 Sep 2008, 00:05
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An integer greater than 1 that is not prime is called composite. If the two-digit integer n is greater than 20, is n composite?

(1) The tens digit of n is a factor of the units digit of n.
(2) The tens digit of n is 2.
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10 Sep 2008, 00:44
dancinggeometry wrote:
An integer greater than 1 that is not prime is called composite. If the two-digit integer n is greater than 20, is n composite?

(1) The tens digit of n is a factor of the units digit of n.
(2) The tens digit of n is 2.

C

the number is even

not a because the number can be 24 or 113
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10 Sep 2008, 04:06
gmatnub wrote:
dancinggeometry wrote:
An integer greater than 1 that is not prime is called composite. If the two-digit integer n is greater than 20, is n composite?

(1) The tens digit of n is a factor of the units digit of n.
(2) The tens digit of n is 2.

C

the number is even

not a because the number can be 24 or 113

That is not true. n is a two digit integer greater than 20....hence A should be sufficient.
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10 Sep 2008, 04:15
IMO A.

(1) The tens digit of n is a factor of the units digit of n >

lets consider the number is 10X +y ; it says Y=kX.. so the number will becose 10X+KX = X (10 +k) which will be a composite number unless X=1 ; now X can not be 1 as 10X+Y> 20

also if you read this option properly the, number shoul be 10X+Y < 49

(2) The tens digit of n is 2.
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10 Sep 2008, 11:46
dancinggeometry wrote:
An integer greater than 1 that is not prime is called composite. If the two-digit integer n is greater than 20, is n composite?

(1) The tens digit of n is a factor of the units digit of n.
(2) The tens digit of n is 2.

Good question!

Basically this question want to know whether n is prime.

Let's assume that n = 10a + b

1) Since b = ax, n = 10a + ax = a*(10+x)
n cannot be prime because it has more than one factor. Or n is a composite. SUFF

2) n = 20 + b
b can be any number from 1 to 9. Some are prime and some are composite. INSUFF
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10 Sep 2008, 11:46
I got A as well but I had to think through it.

(1) The tens digit of n is a factor of the units digit of n.

If 9, factors are 9,3,1 = 19,39,99
If 8, factors are 8,4,2,1 = 18,28,48,88
If 7, factors are 7,1 = 17,77
If 6, factors are 6,3,2,1 = 16,26,36,66
If 5, factors are 5,1 = 15,55
If 4, factors are 4,2,1 = 14,24,44
If 3, factors are 3,1 = 13,33
If 2, factors are 2,1 = 12,22
If 1, factors are 1 = 11

None of the # above 20 are prime, so it is composite.
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10 Sep 2008, 13:10
A

(1) The tens digit of n is a factor of the units digit of n.
ex: any number greater than 20, and tens digit is a factor of the units digit.
24, 26, 28, 36,33....none r prime...so they have to be composite

So, A is sufficient

(2) The tens digit of n is 2.
ex: 21,22,23,24,25....could be prime or composite

So, B is not sufficient...
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10 Sep 2008, 22:07
Good question indeed ... I decided the answer was C before reading through the solutions below... I guess I need to be more careful with DS ....
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11 Sep 2008, 00:30
guys, do we have an OA? A & C are very temptative ...........
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11 Sep 2008, 01:39
OA is A. Bin 4 problem.
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11 Sep 2008, 02:10
What is the source of these problems ?
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14 Sep 2008, 03:53
dancinggeometry wrote:
An integer greater than 1 that is not prime is called composite. If the two-digit integer n is greater than 20, is n composite?

(1) The tens digit of n is a factor of the units digit of n.
(2) The tens digit of n is 2.

1) answers since n=10p+q and p is a factor of q

n=10p+pm div by p
COMPOSITE SUFFI

2)23 is a prime 24 is a composite
INSUFFI
IMO A
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14 Sep 2008, 03:55
dancinggeometry wrote:
OA is A. Bin 4 problem.

Whats the source of these questions ?
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14 Sep 2008, 15:11
spriya wrote:
dancinggeometry wrote:
OA is A. Bin 4 problem.

Whats the source of these questions ?

With A, the values can be only 22,24,26,28,33,36,39,44,48,55,66,77,88,99

All these are composite. We have a Yes, Suff

With B, 21,22,23 We have yes and no. Insuff

hence A
Re: Zumit DS 015   [#permalink] 14 Sep 2008, 15:11
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