Which is the roster form of F₁ = {x : x is a positive multiple of 3 or 5, and x < 20}?
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Answer and explanation
Correct answer: F₁ = {3, 5, 6, 9, 10, 12, 15, 18}
The positive multiples of 3 below 20 are 3, 6, 9, 12, 15, and 18. The positive multiples of 5 below 20 are 5, 10, and 15. Since the condition uses “or,” we take the union of both lists. The common element 15 is written only once because a set does not repeat elements. Thus the complete roster is option A.
Frequently asked questions
What is the correct answer to this question?
F₁ = {3, 5, 6, 9, 10, 12, 15, 18}
Why is this the correct answer?
The positive multiples of 3 below 20 are 3, 6, 9, 12, 15, and 18. The positive multiples of 5 below 20 are 5, 10, and 15. Since the condition uses “or,” we take the union of both lists. The common element 15 is written only once because a set does not repeat elements. Thus the complete roster is option A.
Which subject and chapter does this question cover?
This is a Class 12 Mathematics question. Chapter: Sets.
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