Which number is prime?
- A.51
- B.57
- C.61 ✓
- D.69
Explanation: 51=3×17, 57=3×19, 69=3×23 — all composite. 61 has no factors besides 1 and itself.
Class 6 IMO Mathematics Olympiad practice · Number Sense · Factors, Multiples & Primes
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