Which of the following pairs of numbers is co-prime, even though neither number in the pair is prime?

14 and 21Correct answerYour answer18 and 27Correct answerYour answer22 and 33Correct answerYour answer15 and 28Correct answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer D

Correct answer: D

Two numbers are co-prime when their only common factor is 1.
14 and 21 share 7; 18 and 27 share 9; 22 and 33 share 11.
$15 = 3 \times 5$ and $28 = 2 \times 2 \times 7$ share no prime, so 15 and 28 are co-prime.

Worth noting

Co-prime numbers need not be prime themselves — neither 15 nor 28 is prime.

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