Hard
17 : 23 :: ?
Explanation
Explanation
17 is the 7th prime number and 23 is the 9th prime number - two positions apart in the sequence of primes. Checking the options, 19 is the 8th prime number and 29 is the 10th prime number, which is also two positions apart, matching the same pattern.
PRO TIP
Re-check your working by substituting the answer back into the original question.