The image shows a mathematical set definition:

$B = \{ x \mid x \text{ is a perfect square number between 1 to 20} \}$

To solve this, we need to list the perfect squares between 1 and 20. A perfect square is a number that can be written as $n^2$, where $n$ is an integer.

The perfect squares in this range are:

• $1^2 = 1$
• $2^2 = 4$
• $3^2 = 9$
• $4^2 = 16$

Therefore, the set $B$ is: $B = \{1, 4, 9, 16\}$

Would you like more details or have any questions about perfect squares or set notation? Here are some related questions to expand your understanding:

1. What are the next five perfect squares after 16?
2. How can you find whether a given number is a perfect square?
3. What are the properties of perfect squares?
4. What is the difference between perfect squares and prime numbers?
5. Can negative numbers be perfect squares?

Tip: A quick way to check if a number is a perfect square is to find its square root and check if the result is an integer.