largest factor

Algebra Level pending

A random number is generated by multiplying four positive integers, three of which are consecutive. If two of these numbers are even, what is the largest integer of which this random number is definitely a multiple?


The answer is 12.

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2 solutions

Daniel Xiang
Feb 12, 2018

multiplying the known factors of this random number we get 3 × 2 × 2 = 12 3\times2\times2=12

(it is very easy to see that the product of n n consecutive positive integers is devisible by n ! n! )

Akeel Howell
Feb 12, 2018

The product of any three consecutive integers is a multiple of 3 3 . Since two of the numbers are even, they have prime factorizations that include 2 n 2^n and 2 m 2^m , where m m and n n are not necessarily distinct positive integers. Since the final number is odd and could be anything, none of its prime factors are known.

As such, we see that 2 × 2 × 3 = 12 2 \times 2 \times 3 = 12 is the largest integer of which the random number is a definite multiple.

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