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?
Correct Answer :
The Question can be answered by The using one of the Statements alone, but cannot be answered using the other Statement alone
Solution :
The correct option is: The Question can be answered by using one of the Statements alone, but cannot be answered using the other Statement alone.
Analysis of the Problem:
We are given that there are three distinct prime numbers, let's call them , , and , such that:
(without loss of generality, to keep them distinct)
And their sum is also a prime number, say :
, where is a prime.
Important Property of Primes:
All prime numbers except 2 are odd.
If all three distinct prime numbers , , and were odd, then their sum would be the sum of three odd numbers:
.
This is possible since the sum (which is prime) must also be odd (since the smallest possible sum of three distinct odd primes is , and any prime sum must be greater than 2).
However, if one of the three prime numbers is even (which must be 2, the only even prime), then the sum of two odd primes and one even prime (2) would be:
.
Since the sum must be a prime number greater than 2, it cannot be even.
Therefore, 2 cannot be one of the three prime numbers.
Thus, all three prime numbers , , and must be odd primes.
Analyzing Statement-I: Their sum is less than 23.
We need to find three distinct odd primes such that their sum is a prime and .
Let us test the smallest distinct odd primes: 3, 5, 7, 11, 13, 17, 19.
The minimum possible sum of three distinct odd primes is:
(15 is not prime).
The next possible combinations are:
1) (19 is a prime number).
2) (21 is not prime).
Any other combination of three distinct odd primes will result in a sum greater than or equal to:
(not prime), or (which is not less than 23).
Thus, the only unique set of three distinct prime numbers whose sum is a prime number less than 23 is 3, 5, and 11.
Since we get a unique set of three numbers, Statement-I alone is sufficient to answer the question.
Analyzing Statement-II: One of the numbers is 5.
If one of the numbers is 5, we can have multiple sets of three distinct primes that sum up to a prime. For example:
- Set 1: {3, 5, 11} where the sum is (19 is prime).
- Set 2: {5, 7, 19} where the sum is (31 is prime).
Since there are multiple possibilities for the three numbers, we cannot determine a unique set of three numbers.
Thus, Statement-II alone is not sufficient.
Conclusion:
The question can be answered by using Statement-I alone, but cannot be answered using Statement-II alone.
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