Let R = {x ∈ N : x is a multiple of 3 and x ≤ 100} and S = {x ∈ N : x is a multiple of 5 and x < 100}. What is the number of elements in (R ∩ S) × (S ∩ R)?
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Correct answer: A. 36
Explanation
R ∩ S is the multiples of 15 below 100: 15, 30, 45, 60, 75, 90 — 6 elements. The product has 6 × 6 = 36 pairs.
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