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
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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 \(6\)?
Solution
- 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.
- 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.
- 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.
- 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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