Which of these pairs of numbers is coprime?

A12 and 18

B14 and 21

C15 and 25

D9 and 28

Answer & Solution

Correct answer: D. 9 and 28

1. Find the GCD of each pair. 2. 12 and 18 share 6, 14 and 21 share 7, 15 and 25 share 5. 3. 9 and 28 have GCD 1, so they are coprime. _Source: Wikipedia, Greatest common divisor, section Coprime numbers_

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