B is for Brilliant

In each cell of the following 7 × 7 7\times7 square table, write one of the numbers 1 , 2 , 3 , 4 , 5 , 6 , 7 1,2,3,4,5,6,7 such that the numbers in each row and each column are distinct. What is the greatest possible value of the sum of the numbers written in the blue squares?


The answer is 122.

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5 solutions

Sayan Ghosh
Nov 3, 2013

Let us consider

A 1 A 2 A 3 A 4 A 5 A 6 A 7 \boxed{A1}\boxed{A2}\boxed{A3}\boxed{A4}\boxed{A5}\boxed{A6}\boxed{A7}

B 1 B 2 B 3 B 4 B 5 B 6 B 7 \boxed{B1}\boxed{B2}\boxed{B3}\boxed{B4}\boxed{B5}\boxed{B6}\boxed{B7}

C 1 C 2 C 3 C 4 C 5 C 6 C 7 \boxed{C1}\boxed{C2}\boxed{C3}\boxed{C4}\boxed{C5}\boxed{C6}\boxed{C7}

D 1 D 2 D 3 D 4 D 5 D 6 D 7 \boxed{D1}\boxed{D2}\boxed{D3}\boxed{D4}\boxed{D5}\boxed{D6}\boxed{D7}

E 1 E 2 E 3 E 4 E 5 E 6 E 7 \boxed{E1}\boxed{E2}\boxed{E3}\boxed{E4}\boxed{E5}\boxed{E6}\boxed{E7}

F 1 F 2 F 3 F 4 F 5 F 6 F 7 \boxed{F1}\boxed{F2}\boxed{F3}\boxed{F4}\boxed{F5}\boxed{F6}\boxed{F7}

G 1 G 2 G 3 G 4 G 5 G 6 G 7 \boxed{G1}\boxed{G2}\boxed{G3}\boxed{G4}\boxed{G5}\boxed{G6}\boxed{G7}

Now, A7, D7, G7 are white so we will try to minimize them and allocate them 1, 2,3. To maximize the blue B7, C7, E7, F7, we will allocate them 7,6,5,4

Now, to maximize the blue B1, C1, E1, F1, we will allocate them 4,5,6,7.

Now there are many possible ways to fill up the rest of the table,

We just have to allocate the Ath row(1st) with, 3,4,2,7,6,5,1. 4th row (Dth row) with 1,3,7,6,5,4,2. 7th row (Gth row) with 2,7,6,5,4,1,3.

Adding the values we get 122 \boxed{122}

The matrix I came up with is

3 4 2 7 6 5 1 \boxed{3}\boxed{4}\boxed{2}\boxed{7}\boxed{6}\boxed{5}\boxed{1}

4 5 3 1 2 6 7 \boxed{4}\boxed{5}\boxed{3}\boxed{1}\boxed{2}\boxed{6}\boxed{7}

5 1 4 2 3 7 6 \boxed{5}\boxed{1}\boxed{4}\boxed{2}\boxed{3}\boxed{7}\boxed{6}

1 3 7 6 5 4 2 \boxed{1}\boxed{3}\boxed{7}\boxed{6}\boxed{5}\boxed{4}\boxed{2}

6 2 1 4 7 3 5 \boxed{6}\boxed{2}\boxed{1}\boxed{4}\boxed{7}\boxed{3}\boxed{5}

7 6 5 3 1 2 4 \boxed{7}\boxed{6}\boxed{5}\boxed{3}\boxed{1}\boxed{2}\boxed{4}

2 7 6 5 4 1 3 \boxed{2}\boxed{7}\boxed{6}\boxed{5}\boxed{4}\boxed{1}\boxed{3}

"The table can be filled up in many ways, but you will get the same answer."

I arrived at the same solution! :)

Rindell Mabunga - 7 years, 7 months ago

I did the permutation of making max of 5 lines of 7 boxes and den subractd d least possible sum of 3 uncolord boxes frm d sum of 5 lines 7 boxes. Subtractn is req coz d blank boxes make hole in 3 horizontal lines nd 1 vertical line. And sum of blanck boxes must be least, and least sum possible is 1+2+3 and there are 3 boxes so by permutatn we will do like dis. 5x (1+2+3+4+5+6+7) - 3x (1+2+3) = 122

Shantanu Sinha - 7 years, 7 months ago

I got 123 the only difference between mine and yours is My A1 is 2, now i did this because that is the spot which is the intersection of 2 columns both blue so it makes sense to make it least so i made a7-1 and a4-4 and a5-3,

Pratyush Agarwal - 7 years, 7 months ago

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Because of the symmetry in this figure, even if you interchange the values in the position A7, D7, G7 you are supposed to get the same answer. If I can have a look at your solution then may be we can understand the contradiction!!

SAYAN GHOSH - 7 years, 7 months ago

Why do u need to worry about A4 and A5 ??

Sanjay Banerji - 7 years, 7 months ago

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Really no need to worry about A4 and A5. These can have multiple values.

SAYAN GHOSH - 7 years, 7 months ago
Tjandra Gunawan
Nov 5, 2013

there are 5 line: horizontal up, horizontal middle, horizontal bottom, vertical left, vertical right. each because the line containing permutation 1 to 7, each line sum is (1+2+3+4+5+7) there are 3 intersection 1 in vertical left, and 2 in vertical right (that why there some hole in right). if we want to maximize the sum, the intersection must be as small as possible, each intersection has 3 points: up,middle, and bottom, so the intersection sum is (1+2+3).

answer is: 5x(1+2+3+4+5+6+7)-3x(1+2+3)=5x35-3x6=140-18=122

Nice! I like this argument, except that you'd also have to show that there is an example of arrangement that has this value. As your argument stands, you've proved that the greatest number cannot exceed 122.

David Treeby - 7 years, 7 months ago
Trevor B.
Nov 4, 2013

For this problem, I am going to refer to the rows as A G A \rightarrow G and the columns as 1 7 1 \rightarrow 7 . A group of cells will be notated like in Microsoft Excel; R 1 C 1 R_1C_1 : R 2 C 2 R_2C_2 refers to all of the cells bounded by rows R 1 R_1 and R 2 R_2 and columns C 1 C_1 and C 2 C_2 , inclusive.

There are five major regions of cells that need to be maximized. They are A 1 A1 : G 1 G1 , A 2 A2 : A 6 A6 , D 2 D2 : D 6 D6 , G 2 G2 : G 6 G6 , and B 7 B7 : C 7 E 7 C7 \cup E7 : F 7 F7 .

Obviously, A 1 A1 : G 1 G1 will contain every digit, but the digits can be arranged to maximize A 2 A2 : A 6 A6 , D 2 D2 : D 6 D6 , and G 2 G2 : G 6 G6 . If A 1 A1 , D 1 D1 , and G 1 G1 are minimized, then their rows' blue cells' sums will be maximized.

Looking at the white entries in column 7 7 shows that a maximum sum is achieved when the white entries are 1 1 , 2 2 , and 3 3 in some order (similarly, A 1 A1 , D 1 D1 , and G 1 G1 are 1 1 , 2 2 , and 3 3 in a different order). The blue cells in column 7 7 are 4 4 , 5 5 , 6 6 , and 7 7 in some order.

Putting in numbers as large as possible to fill rows A A , D D , and G G yields that the maximum sums for A 2 A2 : A 6 A6 , D 2 D2 : D 6 D6 , and G 2 G2 : G 6 G6 are 25 \textbf{25} , 24 \textbf{24} , and 23 \textbf{23} . The sum of A 1 A1 : G 1 G1 is 28 \textbf{28} . The sum of the blue cells in column 7 7 is 22 \textbf{22} .

The sum of the entries in the blue entries is 25 + 24 + 23 + 28 + 22 = 122 25+24+23+28+22=\boxed{122} .

An example of a possible board is \downarrow

2654371 7143526 6425137 3761452 4237615 5312764 1576243 2654371\\ 7143526\\ 6425137\\ 3761452\\ 4237615\\ 5312764\\ 1576243

Using LaTeX, is it possible to make text blue? I would have wanted to do that with the bolded numbers, but I couldn't figure out how.

Trevor B. - 7 years, 7 months ago

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To change the color, you can type it as \color{blue} {text}, which gives:

t e x t \color{#3D99F6} {text}

Note that you need { } around all the text, otherwise it will only make the first thing blue, as so:

t e x t \color{#3D99F6} text

Calvin Lin Staff - 7 years, 7 months ago

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T h a n k s ! \color{#3D99F6}{T}\color{#20A900}{h}\color{#EC7300}{a}\color{#D61F06}{n}\color{#69047E}{k}\color{#E81990}{s}\color{grey}{!}

(Admit it! You saw this coming.)

Trevor B. - 7 years, 7 months ago

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@Trevor B. I looked at your Latex code. There wasn't a need to use { } around each letter, since there is only 1 of them in each. For example, 2^3 and 2^{3} both display as 2 3 2^3 . However, there is a big difference between 2^34 and 2^{34}, namely 2 3 4 2^34 and 2 34 2^{34} .

Calvin Lin Staff - 7 years, 7 months ago
Sanjay Banerji
Nov 4, 2013

Think that there are 5 rows or columns blued in total :: (Summing to a total of 28*5 == 140)

Now we are counting the 2 blued corners twice and the 3 non blue squares on the column also twice

We need to substract them and the lowest possible value would be a combination of 1,2,3 with the conditions satisfied..

By this we get the sum as :: (140-(1+2+2)3) == 122

Can you explain the last step? Why do you subtract (1+2+2)3? How does this relate to "1,2,3"?

Can you check that the mathematical statements display correctly? I don't get 122 from your sum.

Calvin Lin Staff - 7 years, 7 months ago
Snehdeep Arora
Nov 5, 2013

Sum of the numbers in the first blue column is 28.In the first,middle and last rows only one number is missing,let's say 1,2,3 in any order is missing. Individual sums 25,26,27. In the four remaining squares only 4,5,6,7 (in any order) can be there.Total sum 28 + 27 + 26 + 25 + 22 = 128 28+27+26+25+22=128 .Now to the three common squares,for the sum to be the greatest those 3 should contain 1,2,3 (in any order). Greatest sum= 128 ( 3 + 2 + 1 ) = 122 128-(3+2+1)=122

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