A = {a, e, i, o, u} ↑ well-defined
This collection of vowels is fixed and well-defined i.e. it does not change from person to person. A set is a well-defined collection of things/entities. ∴ It is a set //
A = {2, 3, 5, 7, 11} ↑ well defined collection
This collection is well-defined and does not vary from person to person. No one can say 4 is prime, as it is false. ∴ This collection is a set // ∵ Set is a well defined collection of things/entities.
A set is a collection of things/entities that are well-defined. We know that India has 28 states. It is fixed and well-defined, as of now. ∴ This collection is a set //
We know there are a total of 15 elected presidents & 3 interim ones. Excluding the current one, we have 14 + 3 = 17 // There is no doubt, it is well-defined. ∴ It is a set // ∵ set is a well-defined collection of entities.
We are sure that the number of such cricketers is well-counted and is well-defined. ∵ A set is well-defined collection ∴ This collection is also well-defined i.e. a set //
Who is good? ↑ To one person, Dhoni is good, to another, Virat is better.
The word "good" is not well-defined. It varies from person to person. ∵ A set must be a well-defined collection. ∴ This collection is not a set //
A = {1, 2, 3, . . . . 49} ↑ This is a well-defined collection, there can be only these numbers in this collection 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 and no OTHER number. ∴ It is a set //
What is "good"? It is not well defined. One player may be good according to a person A, another is good according to a person B. So, this collection is not well defined. => Not a set // ∵ A set is a well-defined collection of entities.
Yes, it is a set. We can count the number of girls in a particular class. It is well-defined. A set is a well-defined collection of things/entities.
For a mother the most talented writer may be her child. For another it may be Premchand. For another younger person it may be Chetan Bhagat. => "most talented" is vague i.e. not well defined. ∴ It is not a set. ∴ False //