Which of these numbers is co-prime with 15?
- A.9
- B.10
- C.16 ✓
- D.20
Explanation: 15 = 3×5. 9 shares the factor 3, 10 shares the factor 5, and 20 shares the factor 5 — but 16 = 2⁴ shares nothing with 15.
Class 6 IMO Mathematics Olympiad practice · Number Sense · Factors, Multiples & Primes
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