Placing Numbers in a Square 1

Logic Level 3

Each of the integers from 1 to 12 is to be placed in one of the circles in the figure so that the sum of the integers along each side of the figure is 25. Determine the sum of the four integers placed in the corners.


Looking for an extra challenge? Try a more difficult version of this problem: Placing Numbers in a Square #2


The answer is 22.

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5 solutions

Kay Xspre
Sep 9, 2015

The sum of all numbers from 1 to 12 is 12 × 13 2 = 78 \frac{12\times13}{2} = 78 , however, this does not take account that the corners were count twice in summing the sum of each side. Given the sum for all sides (when the corner was counted twice) is 25 × 4 = 100 25\times4 = 100 , the corner shall have the sum of 100 78 = 22 100-78 = 22

Same as my way to solve it

Luki Purwoko - 5 years, 9 months ago

Great solution @Kay Xspre

Mehul Arora - 5 years, 9 months ago

Nice solution. But is such an arrangement possible?

Ansh Bhatt - 5 years, 9 months ago

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Example Example

An example is provided one here. There may be other possible arrangement as well.

Kay Xspre - 5 years, 9 months ago

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I wrote a program to find all of the solutions. There are 656 real solutions (once duplicates are accounted for). Only 41 answers exist if you only count unique corners as a solution.

Kendall Belles - 5 years, 8 months ago
Uahbid Dey
Sep 11, 2015

1 + 2 + 3... + 12 = 12 × 13 2 = 78 1+2+3 ... + 12 = \frac{12\times 13}{2}=78 , where, each number counted once. If we add the numbers along sides, e.g. consider this square as ABCD, now if we add the numbers along side AB and side DA, then the number on corner A is counted twice. Similarly if we add up the numbers along 4 sides it will include the 4 corner numbers twice and the sum becomes as 25 × 4 = 100 25\times 4 = 100 . Hence the sum of numbers in 4 corners = 100 78 = 22 =100-78=22 .

Menachem Avinoam
Nov 3, 2015

I brute-forced it with:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
function addup(o)
{
    if(o[0]+o[1]+o[2]+o[3]!=25)return false;
    if(o[8]+o[9]+o[10]+o[11]!=25)return false;
    if(o[0]+o[4]+o[6]+o[8]!=25)return false;
    if(o[3]+o[5]+o[7]+o[11]!=25)return false;
    return true;
}


var o=[1,2,3,4,5,6,7,8,9,10,11,12];
var foundIt=false;
for(var i=0;i<100000;i++)
{
    o.sort(function(){return [-1,1][Math.floor(Math.random()*2)];})
    if(addup(o)){foundIt=true;break;}
}
if(foundIt)
{
    alert('found it!\n'+o.join(','));
}else{
    alert('no luck... try refreshing page');
}

easy. 1+4+7+10=22. hahaha

Chris White
Oct 5, 2015

I came up with arrangement 12 8 4 1 on top, 12 2 5 6 going down left side, 1 10 11 3 going down right side and 6 9 7 3 on bottom.

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