Removing pieces of paper

Number Theory Level pending

Albert has 2 n 2n pieces of paper numbered 1 to 2 n 2n . He removes n n pieces of paper, numbered consecutively. The sum numbers on the remaining pieces of paper is equal to 1615. Find the sum of all possible values of n n .


The answer is 72.

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

Kushal Bose
Feb 15, 2017

Total sum of all pages is 2 n ( 2 n + 1 ) / 2 = n ( 2 n + 1 ) 2n(2n+1)/2=n(2n+1)

Let the papers Albert took is starts with the positive integer k k .Then consecutive n n papers are k , k + 1 , k + 2 , . . . . . , k + n 1 k,k+1,k+2,.....,k+n-1 .Sum of these papers is n k + n ( n 1 ) / 2 nk+n(n-1)/2 .

According to the question n ( 2 n + 1 ) n k n ( n 1 ) / 2 = 1615 n(2n+1) -nk-n(n-1)/2=1615 .Re-arranging the equation

n ( 3 n + 3 2 k ) = 2 × 5 × 17 × 19 n(3n+3 - 2 k)=2 \times 5 \times 17 \times 19

If n n is odd then 3 n + 3 3n+3 is even and 3 n + 3 e v e n = 2 k 3n+3 - even=2k so, 2 k 2k will be even and k k will be an integer. ..But we have to check k + n 1 2 n k+n-1 \leq 2n .And it is easy to check no odd value will be allowable.

If n n is even 3 n + 3 3n+3 is odd and 3 n + 3 o d d = 2 k 3n+3-odd=2k .Again k k will be an integer.Here k k satisfy the above condition that is k + n 1 2 n k+n-1 \leq 2n .So, n = 34 , 38 n=34,38 are only solutions.

Can you write a solution that is helpful to those who cannot solve it? Else I'm inclined to delete this solution to encourage others to contribute a relevant answer.

Brilliant Mathematics Staff - 4 years, 3 months ago

Log in to reply

Okk I am writing

Kushal Bose - 4 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...