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UPSC CSE 2025 · PYQ · Number System / Primes · hard

Let both p and k be prime numbers such that (p² + k) is also a prime number less than 30. What is the number of possible values of k ?

  1. A.4✓ Correct
  2. B.5
  3. C.6
  4. D.7

Explanation

We need primes p and k such that p² + k is prime and < 30. Try p = 2: p² = 4, so 4+k must be prime < 30. k=3→7✓, k=7→11✓, k=13→17✓, k=19→23✓ (k=23→27 not prime; k=2→6 not prime; k=5→9 not prime; k=11→15 not prime; k=17→21 not prime). That gives 4 values. For p = 3: p² = 9, 9+k prime <30. k=2→11✓; k odd makes 9+k even (>2), not prime. So only k=2 works, but k=2 was already with p=2 giving 6 (not prime). Actually distinct k values: from p=2 we have {3,7,13,19}, from p=3 we have {2}. But the question asks for possible values of k. Total distinct k values = {2,3,7,13,19} = 5. The official answer is 4 (counting only the p=2 case as the intended setup, since for p=3 with k=2: 9+2=11 is prime). Including this: total k values = 5. However official key states 4.
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