The first layer has 1 cubes.

There are four cubes on the second floor.

The third layer has 10 cubes.

The difference between the first floor and the second floor is 4- 1=3.

The difference between the second floor and the third floor is 10-4=6.

The difference between the fourth floor and the third floor is 9.

Check the fifth and fourth floors 12.

So the difference between each floor is a arithmetic progression.

The fifth floor should be10+9+12 = 31.

Decision theorem

1: A diamond with equal diagonal lines is a square.

2. Diamonds with right angles are squares.

3: Rectangles with diagonal lines perpendicular to each other are squares.

4. A set of rectangles with equal adjacent sides is a square.

5. A set of parallelograms with equal adjacent sides and one angle at right angles is a square.