Two co-prime numbers, both less than 10, have a product of 21. What are they?
- A.3 and 7 ✓
- B.4 and 5
- C.2 and 9
- D.1 and 21
Explanation: 21 = 3×7, and both 3 and 7 are less than 10 with no common factor. 4×5=20 and 2×9=18 don't give 21, and 21 itself isn't less than 10.
Class 6 IMO Mathematics Olympiad practice · Number Sense · Factors, Multiples & Primes
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