Odd number: the non-prime among primes (or pick the prime)

This type of question, asked 3 times: 12 Sep 2025 S1, 13 Sep 2025 S3, 22 Sep 2025 S3. All 4 questions below, oldest first.

Try each one first, then open its answer.

Question 1
12 Sep 2025 S1
17 $Identify the odd one: 2, 3, 5, 7, 11, 13, 17, 20$
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Question 2
12 Sep 2025 S1
18 $Which of the following is the odd one out?$
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Question 3
13 Sep 2025 S3
4 $Choose the option that is similar to the following numbers:29,31,47, 101$
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Question 4
22 Sep 2025 S3
7 $Identify the number which is different from the other options:$
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Question 1