Which one does NOT belong?

Exercise and reference answers

Questions

1. Items

   1. 9
   2. 2
   3. 3
   4. 7

   Reason

   9 is composite because it is 3 times 3. The other three are prime.

   Odd item

   9

2. Items

   1. 11 x 23
   2. 12 x 21
   3. 2 x 2
   4. 3 x 7 x 11

   Reason

   12 and 21 are not prime, so this is not a prime factorisation. The other three are products of primes.

   Odd item

   12 x 21

Instructions

Tap the item that does not belong with the rest.

Explanation