Which of the following is an example of a set?
*
Lucky numbers that are less than 30.
The group of all natural numbers divisible by 3 and less than 30
The generous faculties of the Math Department
The collection of tall people in the world.

In mathematics, a **set** is a well-defined collection of distinct objects, considered as an object in its own right. The key characteristic of a set is that its elements must be clearly defined, meaning it should be possible to determine whether an object belongs to the set or not.

Let's evaluate each option:

1. **Lucky numbers that are less than 30**: "Lucky numbers" is a subjective term that isn't precisely defined in mathematics. Since "lucky" can vary from person to person, this collection is not well-defined and thus not a proper set.

2. **The group of all natural numbers divisible by 3 and less than 30**: This is a well-defined set because it specifies clear criteria (natural numbers divisible by 3 and less than 30). The elements of this set can be explicitly listed as \{3, 6, 9, 12, 15, 18, 21, 24, 27\}.

3. **The generous faculties of the Math Department**: "Generous" is a subjective term that can vary in interpretation, so this collection is not well-defined. Therefore, it is not considered a set.

4. **The collection of tall people in the world**: "Tall" is a relative term without a precise definition, which makes it subjective. As a result, this collection is not well-defined and is not considered a set.

### Conclusion:
The correct answer is **"The group of all natural numbers divisible by 3 and less than 30"**. This is an example of a well-defined set.

Would you like more details or have any questions?

Here are some follow-up questions:

1. What are some other examples of well-defined sets?
2. How do you represent a set using set-builder notation?
3. What is the difference between a finite and an infinite set?
4. How do you determine the cardinality of a set?
5. What operations can you perform on sets, such as unions and intersections?

**Tip:** Always ensure that the criteria defining a set are clear and unambiguous, so that every object can be easily classified as either a member of the set or not.

Explore examples of sets in mathematics with clear definitions and criteria. Learn about well-defined sets and distinguish them from subjective collections. Suitable for Grades 7-9 students studying set theory.

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