Divisibility problem

Find the greatest positive integer x x such that 2 3 x 23^{x} divides 2000 ! 2000! .


The answer is 89.

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1 solution

Ravi Dwivedi
Jul 17, 2015

The number 23 23 is prime and divides every 23 23 rd number. In all, there are 2000 23 = 86 \lfloor \frac{2000}{23} \rfloor =86 numbers from 1 1 to 2000 2000 that are divisible by 23 23 .

Among those 86 86 numbers, three of them, namely 2 3 2 , 2 2 3 2 , 3 2 3 2 23^2, 2 \cdot 23^2, 3 \cdot 23^2 are divisible by 2 3 2 23^2 .

Hence 2 3 89 2000 ! 23^{89} | 2000! and x = 89 x=\boxed{89}

Moderator note:

It is better to know of the general approach to solving such a problem, instead of "calculate those with 23, and then those with 23^2. How do you know what to calculate, and why that works?

I've updated your problem for simplicity. Can you update your solution accordingly?

Calvin Lin Staff - 5 years, 11 months ago

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