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Q38·CSAT · Prelims 2025

Sets of three integers with LCM 1001, HCF 1

NumericalLCM & HCFNumber theoryHard

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

a

6

b

7

c

8

d

More than 8

Answer

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".

For k numbers with a square-free LCM of N (having p distinct prime factors) and HCF of 1, the number of ordered tuples is (2^k - 2)^p.

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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