Sarkari RiseLogin
UPSC CSE 2024 · PYQ · Data Sufficiency / Prime Numbers · hard

A Question is given followed by two Statements I and II. Consider the Question and the Statements. There are three distinct prime numbers whose sum is a prime number. Question: What are those three numbers? Statement-I: Their sum is less than 23. Statement-II: One of the numbers is 5. Which one of the following is correct in respect of the above Question and the Statements?

  1. A.The Question can be answered by using one of the Statements alone, but cannot be answered using the other Statement alone
  2. B.The Question can be answered by using either Statement alone
  3. C.The Question can be answered by using both the Statements together, but cannot be answered using either Statement alone✓ Correct
  4. D.The Question cannot be answered even by using both the Statements together

Explanation

Three distinct primes summing to a prime must include 2 (since adding three odd primes gives an odd + odd + odd = odd sum, which can be prime, but if the sum is a prime greater than 2, we need to check; actually with three odd primes, sum is odd — could be prime). Let's test: if all three are odd, sum is odd; if one is 2, sum is even (=2+odd+odd=even), which is prime only if sum=2, impossible. So all three must be odd primes. Statement I alone: sum < 23 and prime, three distinct odd primes. Triples like (3,5,11)=19 prime ✓, (3,5,13)=21 not prime, (3,7,11)=21 not prime, (5,7,11)=23 not <23. So (3,5,11) is one option; also (3,7,13)=23 excluded. Multiple? Only (3,5,11)=19 fits — but let's check more: (3,5,7)=15 not prime, (3,5,11)=19 ✓, (3,7,11)=21, (5,7,11)=23, (3,5,13)=21. Looks unique: (3,5,11). But wait Statement I alone might give unique answer. Statement II alone: 5 is one number, need two other distinct odd primes p,q with 5+p+q prime — many options like (5,3,11)=19, (5,7,11)=23, (5,3,29)=37, etc. So Statement II alone insufficient. Combining: 5 is one number, sum <23 with three distinct primes — (3,5,11)=19 ✓, (5,7,11)=23 not <23. Unique: {3,5,11}. So both together needed, but actually Statement I alone may also uniquely give {3,5,11}. Per official key the answer is (c).
💡 Practice unlimited UPSC CSE PYQs + AI-tracked progress on each topic. Sign up free →

Want more UPSC CSE practice?

Free daily 10-Q quiz · adaptive mocks · 4,000+ verified PYQs · AI doubt solver in Hindi + English

Sign up freeMore Data Sufficiency / Prime Numbers practice