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m990540
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Prime Number Question 05 Jan 2011, 18:19
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0% (00:00) correct 100% (00:18) wrong based on 4 sessions

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What is the product of all the prime factors of 18?

A 108
B 18
C 9
D 6
E 2

Thanks for any explanation as to why the answer is what it is!



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B
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Mongolia2HBS
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Prime Number Question 05 Jan 2011, 18:26
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D.
Prime factors: 1,2,3
1*2*3=6
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Prime Number Question 05 Jan 2011, 18:38
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That's what I thought it was too; however, the official answer is B with an explanation of:

The product of all the prime factors of any number is the number itself. Thus, the product of all the prime factors of 18 is 18.

I've never heard of that before and was unsure of the theory behind it.
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Prime Number Question 05 Jan 2011, 18:44
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Well, I was wondering should I multiply by the other 3 or no.
Then prime factors are: 1,2,3,3
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Prime Number Question 05 Jan 2011, 23:10
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+1 for B, the question doesn't say anything about the prime factors being distinct so, prime factors include 1,2,3,3. Product of which is 18
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Prime Number Question 06 Jan 2011, 00:21
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The factors of 18 :
1*18 , 2*9,3*6.

The prime Numbers are 2, and 3.
I believe for this kind of Question we should simply break down 18 into Only Prime Factors whose product would amount to 18. So It can't be 2*2*3 = 12. It can be 2*3*3= 18.

The product of all the prime factors of any number is the number itself

Ex: 12 , factors are 2,3,4,6,1. Now again prime Numbers are Only Two i.e 2 and 3 , this time we would have 2*2*3=12 .

Ex: 9 , factors are 1,3,9. Ans - 3*3 = 9

Hope that helps.

Also 1 is not a prime Number as the rule goes - A prime Number should be Divisible by Itself and 1 , and have two Distinct factors.

Correct me Just In Case.

Best.
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Prime Number Question 06 Jan 2011, 00:39
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m990540
What is the product of all the prime factors of 18?

A 108
B 18
C 9
D 6
E 2

Thanks for any explanation as to why the answer is what it is!
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Prime factors of 18 are just 2 & 3
So answer should be 2x3 = 6
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Prime Number Question 07 Jan 2011, 04:31
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prab
+1 for B, the question doesn't say anything about the prime factors being distinct so, prime factors include 1,2,3,3. Product of which is 18
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I agree except for the list of prime factors, since 1 is not a prime number.
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Prime Number Question 07 Jan 2011, 04:34
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forgot, 1 is not a prime number
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Prime Number Question 08 Jan 2011, 21:50
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One of those debatable questions where the answer could differ based on your interpretation.

How many prime factors does 18 have? Your answer might be 2. It would be How many distinct prime factors does 18 have? Still 2 of course!
What is the product of prime factors of 18? 6 (2*3)
But there could be a debate that the question doesn't say 'distinct prime factors'.

I don't think GMAT will ask you a question where you need to take a call. They will mention 'distinct' or otherwise. And even if they do have such a question, it is highly unlikely that they would retain it since people obtaining scores from 200 to 800 will be giving erroneous answers.

My suggestion is to not worry too much about it.



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