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For any positive integer n, the length of n is defined as the number of prime factors whose product is...

GMAT Number Properties : (NP) Questions

Source: Official Guide
Number Properties
Primes
MEDIUM
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Notes
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For any positive integer \(\mathrm{n}\), the length of \(\mathrm{n}\) is defined as the number of prime factors whose product is \(\mathrm{n}\). For example, the length of \(75\) is \(3\), since \(75 = 3 \times 5 \times 5\). How many two-digit positive integers have length \(6\)?

A
None
B
One
C
Two
D
Three
E
Four
Solution
  1. Translate the problem requirements: We need to find two-digit numbers (10-99) where the total count of prime factors equals 6. For example, \(75 = 3 \times 5 \times 5\) has length 3 because we count each prime factor occurrence.
  2. Identify possible prime factor combinations: Determine all ways to write 6 as a sum of positive integers, where each integer represents how many times a prime appears in the factorization.
  3. Calculate minimum values for each combination: For each combination, find the smallest number by using the smallest possible primes, then check if any variations fall within the two-digit range.
  4. Count valid two-digit numbers: Systematically check which combinations produce numbers between 10 and 99, counting all possibilities.

Execution of Strategic Approach

1. Translate the problem requirements

When we say the length of a number is the count of prime factors, we're counting each prime factor every time it appears.

For example:

  • \(75 = 3 \times 5 \times 5\): length = 3
  • \(12 = 2 \times 2 \times 3\): length = 3

We need two-digit numbers (10-99) with exactly 6 prime factors.

2. Identify possible prime factor combinations

List all partitions of 6 and associate with primes:

  • \(6 = 6\)\(2^6\)
  • \(6 = 5 + 1\)\(2^5 \times 3\)
  • ... (other partitions) ...

3. Calculate minimum values

  • \(2^6 = 64\)
  • \(2^5 \times 3 = 96\)
  • All others exceed 99 ✗

4. Final Answer

Only two numbers: \(64\) and \(96\).

Answer: 2.

Answer Choices Explained
A
None
B
One
C
Two
D
Three
E
Four
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