At the beginning there are $2017$ marbles in each of $1000$ boxes. On each move Aybike chooses a box, grabs some of the marbles from that box and delivers them one for each to the boxes she wishes. At least how many moves does Aybike have to make to have different number of marbles in each box?
Problem
Source: Turkey EGMO TST 2017 P2
Tags: Turkey, EGMO, TST, combinatorics, contest problem
02.06.2017 01:11
Call the marbles that Aybike takes out of boxes "disposable marbles." If Aybike can succeed with $n$ moves, he will have at most $2017n-\frac{n(n-1)}{2}$ disposable marbles, because he will have to leave at least $\frac{n(n-1)}{2}$ total marbles in the boxes he grabbed from so those boxes have differing numbers of marbles. To make the remaining $1000-n$ boxes have differing numbers of marbles, he needs at least $\frac{(1000-n)(999-n)}{2}$ disposable marbles. Thus, $$2017n-\frac{n(n-1)}{2}\ge \frac{(1000-n)(999-n)}{2}$$$$\Longleftrightarrow 4034n-n^2+n\ge n^2-1999n+999000$$$$\Longleftrightarrow 6034n-2n^2\ge 999000$$$$\Longleftrightarrow 3017n-n^2\ge 499500$$$$\Longleftrightarrow n^2-3017n+499500\le 0$$$$\Longleftrightarrow n^2-3017n+\left(\frac{3017}{2}\right)^2\le \left(\frac{3017}{2}\right)^2-499500$$$$\Longleftrightarrow \left(\frac{3017}{2}-n\right)^2\le 1776072.25$$$$\Longleftrightarrow \frac{3017}{2}-n\le \sqrt{1776072.25}$$$$\Longleftrightarrow n\ge \frac{3017}{2}-\sqrt{1776072.25}$$$$n\ge 175.806\dots $$To see that $n=176$ works, take $2018-i$ balls out of the $i$th box for each $1\le i\le 176$, and then put $i$ additional marbles in the $(177+i)$th box for each $0\le i\le 823$. Finally, put $516$ more of these additional balls in box $1000$. Clearly this gives a distinct number of balls in each box, and this is valid because $$(2017+2016+2015+\dots +1842)=(1+2+3+\dots +823)+516$$Thus, the minimum is $\boxed{176}$.
14.12.2017 19:27
I dont understand, you didnt use that there are 1000 boxes. There must be something wrong
15.12.2017 09:51
Thanks - I originally misread it as there being $2017$ boxes. It should be fixed now.
15.12.2017 16:43
I think the min n is 500.Here is the proof If we make n moves there will be at least 1000-n boxes that have never been touched. Also this boxes can take values between 2017 and 2017+n .So n+1>=1000-n. Hence n>=500
16.12.2017 01:34
The problem definitely wasn't translated great, but I think it means to say that Aybike can distribute the balls he takes out to many different boxes on a single move. Otherwise the problem would be pretty silly and as you said the answer would simply be $500$.