Sets of three integers with LCM 1001, HCF 1
Question
There are n sets of numbers each having only three positive integers with LCM equal to 1001 and HCF equal to 1. What is the value of n?
Options
6
7
8
More than 8
Explanation
The prime factorization of 1001 = 7 × 11 × 13. For a set of three numbers \{a, b, c\}, the LCM dictates the maximum exponent of each prime factor must be 1, and the HCF dictates the minimum exponent must be 0. For each prime, the valid exponent triplets (e_a, e_b, e_c) that contain at least one 1 and at least one 0 are: (1,0,0), (0,1,0), (0,0,1), (1,1,0), (1,0,1), (0,1,1). This gives 6 ordered arrangements per prime. Total ordered triplets = 6 × 6 × 6 = 216. To find unordered sets of distinct numbers, we remove cases where two numbers are identical (24 cases) and divide by 3! (6 arrangements). Unordered sets = (216 - 24) / 6 = 32. Since 32 is strictly greater than 8, the answer is "More than 8".
Answer: (d).
Question details
Year
2025
Paper
CSAT
Question
Q38
Section
Numerical Ability
Sub-topic
LCM & HCF
Type
Number theory
Difficulty
Hard
Source hint
Number theory
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