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Q25·CSAT · Prelims 2025

N as sum of three distinct factors below 50

NumericalFactors & DivisorsNumber theoryHard

Question

A natural number N is such that it can be expressed as N = p + q + r where p, q and r are distinct factors of N. How many numbers less than 50 have this property?

Options

a

6

b

7

c

8

Answer
d

9

Explanation

Divide the given equation N = p + q + r by N to get: 1 = p/N + q/N + r/N. Since p, q, r are factors of N, the fractions N/p, N/q, N/r must be distinct integers. Let's call them P, Q, R. We get 1 = 1/P + 1/Q + 1/R. The only solution to this equation with distinct positive integers is 1 = 1/2 + 1/3 + 1/6. This means the factors must be ⟨MATH⟩N/2, N/3, N/6⟨/MATH⟩. For these to be integers, ⟨MATH⟩N⟨/MATH⟩ must be a multiple of 6. The multiples of 6 strictly less than 50 are: 6, 12, 18, 24, 30, 36, 42, 48. There are 8 such numbers.

The equation x = a + b + c where a, b, c are distinct factors of x is a classic property of multiples of 6, derived from the unit fraction identity 1/2 + 1/3 + 1/6 = 1.

Answer: (c).

Question details

Year

2025

Paper

CSAT

Question

Q25

Section

Numerical Ability

Sub-topic

Factors & Divisors

Type

Number theory

Difficulty

Hard

Source hint

Number theory

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