Move 3 sticks in the shape made up of 15 sticks and form 2 squares
The Stick Puzzle
Part I: Move 3 matchsticks in the figure shown and make 2 squares.
Part II: How many unique solutions can you find? A unique solution means—if you rotate the solution figure in any way, it will still remain unique. And you have to find ALL such unique solutions.
Total recommended time: 5 minutes.
The matchstick puzzle figure is not made up of complete regular geometric shapes of squares or triangles. The figure is not a bounded shape. And it consists of a large number of 15 sticks.
How should you proceed?
You can go ahead by using trial and error, but usually that takes more time. Instead, if you do a bit of analysis, you can reach at least one solution quickly. And that'll be the solution to the first part of the puzzle.
Solution: Structural analysis and reasoning—Identifying which 3 sticks to move
First step of solving any matchstick puzzle is to count number of sticks—it is 15.
Obviously, to make 2 squares out of these 15 sticks,
The 2 squares must be of unequal size—one large square with 3 stick side length and one small square of 1 stick side length.
This is the first conclusion.
Why not 4-stick side larger square?
Four-stick long sides of a square would require 16 sticks. Even if you create the 1-stick side smaller square attached in a corner of the larger square, number of common sides will be 2, and total number of sticks required would be $16+4-2=18$, more than what you have.
And if you attach the smaller square anywhere else inside the larger square, number of common sides will be only 1, and total number of sticks used would further increase by 1 to 19.
Think over: Why you cannot use 3-stick side larger square and 2-stick side smaller square.
How can you use 15 sticks to form two squares?
Number of sticks required to make two such independent squares would be, $12+4=16$.
As you have 15 sticks,
There must be 1 stick common between the two squares reducing the number of sticks needed from 16 to 15.
This is the second conclusion.
The third conclusion is easy to make,
You have to close the gap of the larger square, AND form the 1-stick side smaller square attached to the middle stick of any side of the large square.
If you form the small square at any corner of the large square, there would be 2 common sticks, not 1. Sticks used would then be 14 not 15.
Which sticks to move?
Candidates obviously are the four sticks inside the larger square. You may keep stick 1 undisturbed, but sticks 2, 3 and and 4 you must move as these are well inside the larger square.
The following figure shows the four candidate sticks numbered.
Which 3 of the four sticks are to be moved?
Final stage: Movement restrictions on stick 1 forces solution
You have already concluded that the new smaller square must be formed attached to the middle stick of a side of the larger square (so that number of common side is 1, not 2).
You have stick 1 already waiting for the other two sides of the smaller square to be formed. So it cannot be moved.
Solution is now clear,
Move any of the 3 sticks numbered 2, 3 and 4 to close the gap of the larger square, and form the smaller square with the rest two free sticks and already existing stick number 1.
Following figure shows the solution.
How many unique solutions?
For a moment forget the restriction of keeping stick 1 unmoved.
If instead of stick 1, you keep stick 2 fixed can you not make another similar solution shape?
Why, yes. You can.
Close the gap of the large square by stick 1 and complete the two other sides of small square by moving stick 3 and stick 4 with stick 2 already waiting as an existing side.
The small square is formed in this case on the left vertical side of the large square.
This will be the second solution.
Yes, you can form two other solution shapes by keeping stick 3 and stick 4 respectively.
With stick 3 fixed, the small square is formed on the bottom horizontal side and with stick 4 fixed, the small square is formed on the right vertical side of the larger square.
There can then be four solutions that look different.
If you don't rotate, you will have 4 unique solutions, but if you rotate any of the solution shapes it'll be same as the other three.
Rotationally unique solution is only 1.
You have taken care of all possibilities! Exhaustively.
If you form your own matchstick puzzle and solve it exhaustively using all methods you know and can create, it will be a richly rewarding experience as well as interesting pastime.
Puzzles you may enjoy
Logic analysis puzzles
River crossing puzzles
Ball weighing puzzles
Second Move 3 sticks and make 2 squares matchstick puzzle