In the multiplication of the same two numbers we can get interesting things. In an interesting way, we will be able to make children feel happy knowing that Mathematics is fun. At least we can use it as an intermezo material when children feel bored with learning.
The square of the number with the last digit 5 ​​can be found in several ways. It can be the long way or the short way.
The long way is to multiply normally. What is commonly done is to multiply the stack down. While the quick way can be formulated with

In the figure above, the result of the square can be divided into two parts. The first part is represented by a square and the second part is the number 25. In the box, it is filled in by multiplying the first digit by the first digit plus 1.

Let's look at the following example.
Example 1.
The square of 55 is...
We can do this by multiplying 55 by 55 as in the image below.

If we use the long way then we can do it by multiplying stacking down.

The result of calculating the square of 55 is 3025.
Well, now let's use the quick way.
The number that is squared has two digits. The first digit is five and the second digit is also 5.
The method:
Multiply the first digit by the first digit plus 1 so that it becomes a multiplication of 5 and 6 . Add 25 after the multiplication result.

Example 2.
The square of 75 is...

With these two examples we can determine the square of the other number as follows

That's how to teach the square of numbers with unit digits 5. In this way, it is hoped that children will feel happy learning mathematics.
Thus my writing this time may be useful.

Best regard from Indonesia.
@rokhani

Thank you for stopping by and reading my post. I hope you enjoy it. Upvote, comment, reblog and other support are highly appreciated.