There is a room of dimension $2\times 12$ units. You have to tile it. There are two types of tiles:

  • I-Shaped - it is of a dimension $1\times 2$ units,
  • L-Shaped - it is in the shape of $L$ which has an area of $3$ sq. units.

enter image description here

In how many ways can you tile it?

I have studied some basics of permutation and combination but I can't use it in the this question.


  • The $I$ shaped tiles are identical from both sides.
  • You can rotate the tiles in any direction if it will fit.
  • You have an unlimited supply of tiles.
  • $\begingroup$ What have you tried so far? $\endgroup$ – Vinyl_cape_jawa Feb 23 at 15:05
  • $\begingroup$ I couldn't come to any solution $\endgroup$ – Abhinav Feb 23 at 15:08
  • $\begingroup$ I don't know how to start $\endgroup$ – Abhinav Feb 23 at 15:08
  • $\begingroup$ Maybe you can try doing this recursively, like A[12]=A[11]+A[9], and I think you need a B to tell about if one block is stuck out, like this(can anyone tell me how to change the line please) $\endgroup$ – Math Lover Feb 23 at 15:09
  • $\begingroup$ The area you need to tile is not big at all, try out some $\endgroup$ – Vinyl_cape_jawa Feb 23 at 15:09

I am not quite sure if you can use basic permutation/combination to solve this problem.

But It can be approached recursively.

Denote the number of ways of tiling a $2\times n$ area by given tiles as $T(n)$, and the number of ways of tiling a $2 \times n$ area plus a leading block on top or bottom as $S(n)$


$T(n)=T(n-1)+T(n-2)+ 2S(n-2)$


Take the first relation for example. If you lay the first tile as 'I' and vertically, you result in a $2\times (n-1)$ area. If you lay the first tile 'I' horizontally, you will have to cover the area below it with another horizontal 'I' to cover fully, you get a $2\times (n-2)$ area. If you lay a 'L', you can do it in two valid way, and by symmetry, they have same number ways of being tiled, names $S(n-2)$.

By solving the recurrence relation, you can get the answer to $T(12)$, i.e., your question.



Since the are you need to tile is not that big at all you could easily just list all the tilings (which is definitely not a pretty way to solve it but if you have no idea how to start it at least gives you a starting point).

Using an odd number of $L$ shaped tile you won´t succeed so you can directly start checking the cases where you have an even number of $L$ tiles.

Using $0$ of the $L$ tiles you will be able to tile it using only the $I$ shaped ones and this can be done in only $1$ way.

Using $2$ of the $L$ shaped ones you have $4$ cases,

1) they are next to each other

2) they have $2$ of the $I$ shaped ones inbetween them

3) they have $4$ of the $I$ shaped ones inbetween them

4) they have $6$ of the $I$ shaped ones inbetween them

Observe that in each of these cases you get a factor of $2$ in the number of solutions since each of these can be flipped around a horisontal axis.

And so on...

As I said this is definitely not a nice solution, creating a recurrence relation is a lot nicer altough I am unsure if you know how to use recurrence relation to solve this. Please let me know if you do, in that case we could work out something more mathematical :)

Hope this helped.

  • $\begingroup$ I have another idea. Can we make groups of 2*2 boxes with 2 I-shaped tiles. By P&C we can make that total number of combinations would be $2^6$.Similarly for 3*2 box and so on. After we complete upto 6*2 box we can add the earlier combinations. $\endgroup$ – Abhinav Feb 23 at 16:12

Let $A_n,B_n,C_n,D_n$ denote the following four shapes, where $n$ is the width of the shape. Let $a_n,b_n,c_n,d_n$ be the number of ways to tile them, respectively.

enter image description here

The top right field of $A_n$ can be covered by a vertical $I$, by a horizontal $I$, by the apex of an $L$, or by the "toe" of an $L$. Removing this tile produces either the shape $A_{n-1}$, $D_n$, $B_{n-1}$, or $C_{n-1}$, respectively. We conclude $$\tag1a_n=a_{n-1}+d_n+b_{n-1}+c_{n-1}.$$ The rightmost field of $B_n$ can be covered by a horizontal $I$ or by the "toe" of an $L$. Removing this tile produces either $C_{n-1}$ or $A_{n-2}$, respectively. We conclude $$\tag2b_n=c_{n-1}+a_{n-2}.$$ The rightmost field of $D_n$ can only be covered by a horizontal $I$. We conclude $$\tag3d_n=a_{n-2}.$$ And by symmetry, we clearly have $$ \tag4c_n=b_n.$$ Using $(3)$ and $(4)$ to eliminate $c_n$ and $d_n$, we find $$ a_n=a_{n-1}+a_{n-2}+2b_{n-1}$$ and $$a_{n+1}=a_n+a_{n-1}+2b_n=a_n+a_{n-1}+2(b_{n-1}+a_{n-2})$$ and from these eliminate $b_{n-1}$: $$a_{n+1}= 2a_n+a_{n-2}.$$ This recursion is good enough to quickly compute the desired value $a_{12}$ by hand from the readily obtained starting values $$a_0=1, \quad a_1=1, \quad a_2=2.$$ (The recursion can also be used to ultimately obtain an explicit formula for $a_n$ for arbitrary $n$)


Here is a combinatorial proof of the following recurrence mentioned at the end of Hagen von Eitzen's answer. $$ a_n=2a_{n-1}+a_{n-3} $$ To specify a tiling of a $2\times n$ rectangle, start with a tiling of a $2\times (n-1)$ board, add a vertical $I$ tile to the left end, then do one of two things:

  • Leave it as is.

  • Make whichever of the below replacements is possible, where the xs mark the $I$ tile at the left end.

X Y    ->  X X
X Y        Y Y

X L L  -> L X X
X L       L L

X L    -> L L
X L L     L X X

X Y Y  -> L M M
X Z Z     L L M

This generates almost all possible $2\times n$ tilings. The only ones remaining are the ones ending in

L M M,

which are accounted for by the $a_{n-3}$.


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