Tetromino Dilemma

If I have the 5 colored shapes shown that I can rotate, and I use each shape once, is it possible to place them so they fit perfectly in a 5 × 4 5 \times 4 rectangle?

Hint : Notice the checkerboard pattern of the rectangle, and consider how the shapes would be colored if they followed the same pattern.

Yes No

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16 solutions

Laura Gao
Feb 25, 2018

Relevant wiki: Coloring (Parity Arguments)

There are 10 10 black squares and 10 10 white squares on the board. All the shapes except for the orange one covers 2 2 black squares and 2 2 white squares when placed on the board, regardless of how it is oriented. The orange shape covers 3 3 squares of one color and 1 1 square of the other color. Adding all the shapes up will result in a different number of black and white squares, which is not how the board is. Therefore, it is impossible to cover the board with these five tetrominoes.

So, if we removed the tile of this type, would we be able to cover the board?

Agnishom Chattopadhyay - 3 years, 3 months ago

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@Agnishom Chattopadhyay Actually this answer shows why its not possible with T shape. But it says nothing about your question. After some trial, you will see it will cover.

Samjoe Army - 3 years, 3 months ago

Beautiful!

Me Been - 3 years, 3 months ago

Inteligente!!!

Chante Benjamin - 3 years, 3 months ago

Brilliant!

B NA - 3 years, 3 months ago

A beautiful solution - thanks 👍

John Bament - 3 years, 2 months ago

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you're welcome :)

Laura Gao - 3 years, 2 months ago

This solution was a no brainier.

Jean Chery - 3 years, 1 month ago

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Then why did 38% of solvers get this problem wrong?

Laura Gao - 3 years, 1 month ago
Jason Dyer Staff
Feb 20, 2018

Consider the rectangle colored like a checkerboard. Each tetromino covers exactly two white and two black squares except for the "T" shape which must cover either three white squares & one black square or three black squares & one white square. Since an equal number of black and white squares must be covered to include the complete checkerboard, it is impossible to fit the pieces in a 5 by 4 rectangle.

Moderator note:

While this problem has an elegant solution, the general problem of tiling a rectangle with polyominos can be very hard.

For example, using just copies of the polyomino below, it is an open problem whether they can tile a rectangle (the answer is thought to be "no").

Top notch strategy 👌👌

Jonno Lamboli - 3 years, 3 months ago

what a 'brilliant' clue!

Peter Macgregor - 3 years, 3 months ago

Do you play Blokus Jason? All those shapes match with those in Blokus!

Anuj Shikarkhane - 3 years, 3 months ago

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I'm familiar with the game; Blokus has every polyomino of order 1, 2, 3, 4, and 5.

Jason Dyer Staff - 3 years, 3 months ago

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What is that game about??

Chante Benjamin - 3 years, 3 months ago

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@Chante Benjamin https://en.wikipedia.org/wiki/Blokus

Anuj Shikarkhane - 3 years, 3 months ago

Number of Black and White blocks are not equal, therefore its not possible to fit the 4X5 frame as is.

Tam Nolan - 3 years, 3 months ago
Zain Majumder
Feb 25, 2018

Relevant wiki: Coloring (Parity Arguments)

The green, purple, red, and blue shapes must cover 2 2 white squares and 2 2 black squares each when placed on the board, for a total of 8 8 black and 8 8 white squares. There are now 2 2 black and 2 2 white squares remaining. However, the orange shape cannot be oriented to cover 2 2 black and 2 2 white squares. Therefore, it is impossible to cover the board.

I drew the diagram and then I colored in the different shapes into the diagram and discovered that they didn't fit in. I would rate this problem a 1 because it was really easy to figure out how to do it. At first I thought that i was going to be a bit more challenging but as i looked at it carefully, I noticed that it wasn't.

Lucia Tiberio - 3 years, 3 months ago

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I disagree. You know that one specific arrangement wouldn't fit in. How do you prove that no arrangement would?

Agnishom Chattopadhyay - 3 years, 3 months ago

I don't think the phrase "The green, purple, red, and blue shapes must cover 2 white squares and 2 black squares each" has to be true; That is if you had a grid with even tiles, and (n) shapes total and (n-2) shapes covering the same white tiles as they are covering black ones but 2 shapes covering odd tiles, then (the black tiles covered by either shape and the black tiles covered by the other shape) must add to an even number, the same condition must be true regarding the white tiles too for the grid to be covered entirely. The conditions are necessary to cover the grid, but not the other way around. Also 2 is just even numbe; (n-2) must be (n-2a) blocks is the general rule where a is an integer. https://imgur.com/q8UZef7 created via (http://print-graph-paper.com/virtual-graph-paper)

If I sounded confident then I'm probably wrong. Correct me if I'm so.

Reda Ali - 3 years, 3 months ago

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Maybe I am misinterpreting what you are trying to say, but that does not contradict anything I said. It is true that if two shapes covered an odd number of a color, they would combine to cover an even number. However, there is only one shape (the orange shape) that covers an odd number of a color. The remaining shapes cover exactly 2 2 of each color no matter what. Therefore, the grid cannot be covered.

Zain Majumder - 3 years, 3 months ago
Rakesh Mavuram
Feb 26, 2018

In a 5×4 it has 20 square boxes but in the given shapes has only 18 in total

That was the criteria I Considered as the determination of the problem as presented. 1st I was visually stacking them, then it occurred to me that there was a difference in inventory and warehouse real estate, as we may say. Then it became a question of what constituted the problems required answer as the term "perfect" fit. Open spaces within the "warehouse" didnt seem to fit the description of perfect, regardless of how neatly or symmetrically they got packed in there. So logic directed me to choose (no)

Russ Reed - 3 years, 3 months ago

Uh, what? The rectangle is 5x4, yes, but I don't know where you're getting the 18 from. Tetrominoes (the shapes used in Tetris, and also in this problem) always have four squares. Hence the 'tetra.' So it is physically impossible for them to add up to 18 squares. There are five shapes, each with four squares, therefore there are 20 squares in total across the shapes.

Zanthor Nolastname - 3 years, 2 months ago
Rudy M
Feb 26, 2018

I thought of this like Tetris...

Bill Meister
Mar 3, 2018

Just look at the shape with a single protruding box. You can intuitively conclude that it is incompatible with the rest of the shapes in filling the whole board just by visual comparison.

Consider the rectangle colored like a checkerboard. Each tetromino covers exactly two white and two black squares except for the "T" shape which must cover either three white squares & one black square or three black squares & one white square. Since an equal number of black and white squares must be covered to include the complete checkerboard, it is impossible to fit the pieces in a 5 by 4 rectangle.

Ariijit Dey
Mar 3, 2018

Clearly, there are only six singular pieces or (2 mod 4) anyone you wish so it is not possible to fit this into the rectangle.

Arijit Dey
Mar 3, 2018

Clearly, there are only six singular pieces or (2 mod 4) anyone you wish so it is not possible to fit this into the rectangle

Joyce Lee
Mar 1, 2018

Lets say that its a fraction if you shade in all the parts given to you then it will not fit in all the spaces.

Champa C
Mar 1, 2018

Total count of squares in shapes and total count of squares in the 5X4 box is not same..so the answer is NO

Game Hacker
Mar 1, 2018

no is the awanser

i totally understand it now thanks to your explanation

Rune Knappskog - 3 years, 3 months ago
Dokter Medic
Feb 28, 2018

No need to overcomplicate things. The T shaped piece would leave a gap somewhere on the board. Simply put, if on the bottom, no piece juts down, if on the top, there is no one black piece to fill it, and if in the middle, same problem as with the bottom. Also, even taking into account rotation, there would still end up a gap.

The checkerboard has equal number of black and white sqares All shapes except the Orange when placed would cover an equal number of black and white squares...

But the Orange Shape however placed will cover 2 extra squares of some colour...but the board has an equal number of black and white squares that are to be covered.

Therefore it is not possible to cover it using 1 of each of the coloured shapes given.

The box contains 20 individual smaller boxes. The pieces in the solution contains 21 small individual boxes collectively. There is no way to fit 21 boxes into a space that only contains 20 boxes. The answer is "no"

There are 5 pieces, each of which have 4 individual boxes, so the total is 20, not 21.

Calvin Lin Staff - 3 years, 3 months ago
Julia Seidel
Feb 25, 2018

The trick seems to be to imagine the checkerboard in the tile and not trying to place them on the board in your mind.

My intuition was something like "There too many tiles art facing upwards...creating holes in the mosaic...not depending on how you'd combine them."

Maybe it's about not being able to create a 5x4 rectangle out of the tetris tiles.

There needs to be a certain kind of symmetry, which is probably easiest done dividing the piles into black and white subpieces.

Anyway, I could also use a scissor to divide the ones that aren't fitting - or change the checkerboard...or the rules of the question.

If there doesn't seem to be a solution you haven't thought about the problem enough yet, imo. ;)

Also, I thought that a checkerboard is defined as a 8x8 board? And it's not a square...

But how do you rigorously show that such a tiling is impossible?

Agnishom Chattopadhyay - 3 years, 3 months ago

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