Which number is the odd one out: 51, 57, 61, 87?

51Correct answerYour answer57Correct answerYour answer61Correct answerYour answer87Correct answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer C

Correct answer: C

$51 = 3 \times 17$ , $57 = 3 \times 19$ and $87 = 3 \times 29$ , so these three are composite (each is divisible by 3).
61 has no factor other than 1 and 61, so it is the only prime and is the odd one out.

Worth noting

51, 57 and 87 look prime, which is the trap. Test divisibility by 3 with the digit sum ( $5 + 1 = 6$ , $5 + 7 = 12$ , $8 + 7 = 15$ ).

Concept: Odd one out: primes that look composite and composites that look prime
Typical timeTypical: 70 s
Your timeYours: —

Ten questions from Analogies and Classification, at your own pace.

Ten questions from Analogies and Classification, at your own pace.

Explanation