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If p is a prime number and n is a positive integer, what is the number

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If p is a prime number and n is a positive integer, what is the number  [#permalink]

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New post 25 Jul 2018, 00:08
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[Math Revolution GMAT math practice question]

If \(p\) is a prime number and \(n\) is a positive integer, what is the number of factors of \(3^np^2\)?

1)\(n = 4\)
2) \(p > 4\)

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If p is a prime number and n is a positive integer, what is the number  [#permalink]

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New post Updated on: 25 Jul 2018, 00:31
1
To find the number of factors of \(3^n p^2\)

Statement 1

n = 4

If p \(\neq\) 3 then number of factors = (n + 1) * (2 + 1) = 5 * 3 = 15

If p = 3 then number of factors = n + 2 + 1 = 7

Statement 1 is not sufficient

Statement 2

p > 4

number of factors of \(3^n p^2\) = (n + 1) * (2 + 1)

Since we don't know the value of n, statement 2 is insufficient

Combining statements 1 and 2

p > 4 => p \(\neq\) 3 then number of factors = (n + 1) * (2 + 1) = 5 * 3 = 15

Hence option C
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Originally posted by workout on 25 Jul 2018, 00:17.
Last edited by workout on 25 Jul 2018, 00:31, edited 2 times in total.
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If p is a prime number and n is a positive integer, what is the number  [#permalink]

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New post 25 Jul 2018, 00:27
Given: n=4, p=prime number. To know the number of factors, we need to know the powers of primes and whether p=3.

1) If p=3, then the total number of factors= 6+1 =7. But if p is not equal to 3, then the total number of factors will be (4+1)*(2+1) = 5*3 = 15. Not Sufficient.

2) If p>4, then the total number of factors = (2+1)*(n+1). But we do not know the value of n. Hence not sufficient.

Combining 1 & 2, we get n=4 and p is not equal to 3. Therefore the total number of factors = (4+1)*(2+1) = 15. Sufficient. Therefore, C.
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Re: If p is a prime number and n is a positive integer, what is the number  [#permalink]

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New post 25 Jul 2018, 03:10
To count the factors of a positive integer:
1. Prime-factorize the integer
2. Write the prime-factorization in the form \((a^p)(b^q)(c^r)\)...
3. The number of factors = \((p+1)(q+1)(r+1)\)...

MathRevolution wrote:
[Math Revolution GMAT math practice question]

If \(p\) is a prime number and \(n\) is a positive integer, what is the number of factors of \(3^np^2\)?

1)\(n = 4\)
2) \(p > 4\)


Statement 1: \(n=4\)
Test one case that also satisfies Statement 2.
Case 1: \(p=5\), with the result that \(3^np^2 = (3^4)(5^2)\)
Adding 1 to each exponent and multiplying, we get:
Number of factors \(= (4+1)(2+1) = 15\)

Test a case that does NOT also satisfy Statement 2.
Case 1: \(p=3\), with the result that \(3^np^2 = (3^4)(3^2) = 3^6\)
Adding 1 to the only exponent, we get:
Number of factors \(= 6+1 = 7\)

Since the number of factors can be different values, INSUFFICIENT.

Statement 2: \(p>4\)
Case 1 also satisfies Statement 2.
In Case 1, the number of factors = 15.

Case 3: \(p=5\) and \(n=2\), with the result that \(3^np^2 = (3^2)(5^2)\)
Adding 1 to each exponent and multiplying, we get:
Number of factors \(= (2+1)(2+1) = 9\)

Since the number of factors can be different values, INSUFFICIENT.

Statements combined:
As illustrated by Case 1, if \(n=4\) and \(p>4\), the number of factors = 15.
SUFFICIENT.


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Re: If p is a prime number and n is a positive integer, what is the number  [#permalink]

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New post 26 Jul 2018, 23:49
=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question.

If a question asks for a number of factors, it is very important to check that all of the given prime factors are “different”. By condition 2), \(p\) is a prime number different from \(3\). To determine the number of factors, we need to know the exponents in the prime number factorization. Therefore, we also need condition 1).

Since \(p\) is a different prime integer from \(3\), and \(n = 4\), the number of factors of \(3^np^2\) is \((4+1)(2+1) = 15.\)
Since we have a unique solution, both conditions together are sufficient.

Therefore, C is the answer.
Answer: C
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Re: If p is a prime number and n is a positive integer, what is the number &nbs [#permalink] 26 Jul 2018, 23:49
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