SAT Counter-Examples

Algebra Level 3

If p p is a prime number, which of the following must be true?

I. p is odd. II. p is not divisible by 6. III. p is not divisible by 7. \begin{array}{r r l} &\text{I.} & p\ \text{is odd.}\\ &\text{II.} & p\ \text{is not divisible by}\ 6.\\ &\text{III.} & p\ \text{is not divisible by}\ 7.\\ \end{array}

(A) \ \ I only
(B) \ \ II only
(C) \ \ I and II only
(D) \ \ I and III only
(E) \ \ II and III only

A B C D E

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2 solutions

Sajjad Sajjad
Mar 28, 2015

For p p we can write,

p = 2 , 3 , 5 , 7 , 11 , 13 , p = {2, 3, 5, 7, 11, 13, \dots}

Here p = 2 p=2 is an Even Number.

So, [ I ] [I] is wrong.

Again, p = 7 p = 7 can divide by 7 7

So, [ I I I ] [III] is wrong.

We know, 6 6 is an even number and its mutiples will always be an even.

Hence, p p isn't divisible by 6 6 .

Thus the right statement is [ I I ] [II] and the answer is B B

Tatiana Georgieva Staff
Mar 27, 2015

Correct Answer: B

Solution:

We analyze each of the options.

I. p = 2 p = 2 is a prime number that is not odd. Hence option I need not be true.

II. A prime number has two factors: 1 and itself. If p p is divisible by 6, then it has at least four factors: 1, 2, 3 and 6. Hence, the prime number p p cannot be divisible by 6. Option II is true.

III. p = 7 p = 7 is a prime number that is divisible by 7. Hence option III need not be true.

Because the only true statement is option II, the answer is (B).



Incorrect Choices:

(A) , (C) , (D) , and (E)
The solution explains why these choices are wrong.

If you got this problem wrong, you should review SAT Factors, Divisibility, and Remainders .

1 is the first prime.

Braulio Balbuena Moreno - 6 years, 2 months ago

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I is neither prime nor composite. By definition of a prime, it should have one factor other than one. Hence it is not prime. It is not composite since it has no more factors other than one and itself.

Veeraragavan Subramaniam - 6 years, 2 months ago

I thought we wete supposed to be giving a counter example. Pp number can never be divisible by 6 , but it can be divided by 7 and doesnt have to be odd. You have totally confused me

Peter Farris - 4 years, 3 months ago

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