Which pairs of numbers are relatively prime? Check all that apply.

Relatively prime numbers have no common factor other than 1. The pair 9 and 40 is relatively prime: 9 has factors 1, 3, and 9, while 40 is not divisible by 3. The pair 21 and 40 is also relatively prime because 21 = 3 × 7 and 40 = 2³ × 5, so they share no prime factors. The other pairs share a factor: 9 and 15 share 3, 9 and 21 share 3, and 15 and 21 share 3.

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