2016/2017

Let S S be the set of all positive integers between 1 and 2017, inclusive. Suppose that the least common multiple of all elements in S S is L L . Find the number of elements in S S that do not divide L 2016 \dfrac L{2016} .


The answer is 44.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Patrick Corn
Nov 24, 2017

We have L = 2 10 3 6 7 3 P , L = 2^{10} \cdot 3^6 \cdot 7^3 \cdot P, where P P is a product of other prime powers. Then L / 2016 = 2 5 3 4 7 2 P . L/2016 = 2^5 \cdot 3^4 \cdot 7^2 \cdot P. So we are looking for numbers which are divisible by 2 6 , 2^6, or 3 5 , 3^5, or 7 3 . 7^3. No number 2017 \le 2017 is divisible by two of these simultaneously, so we can count each one separately.

There are 2017 / 2 6 = 31 \lfloor 2017 / 2^6 \rfloor = 31 numbers divisible by 2 6 , 2^6, 2017 / 3 5 = 8 \lfloor 2017 / 3^5 \rfloor = 8 numbers divisible by 3 5 , 3^5, and 2017 / 7 3 = 5 \lfloor 2017 / 7^3 \rfloor = 5 numbers divisible by 7 3 , 7^3, so the answer is 31 + 8 + 5 = 44 . 31 + 8 + 5 = \fbox{44}.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...