Question Details

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?

Options

A

The Question can be answered by The using one of the Statements alone, but cannot be answered using the other Statement alone

B

The Question can be answered by using either Statement alone

C

The Question can be answered by using both the Statements together, but cannot be answered using either Statement alone

D

The Question cannot be answered even by using both the Statements together

Show Answer

Correct Answer :

Option A

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 p, q, and r, such that:
p<q<r (without loss of generality, to keep them distinct)
And their sum is also a prime number, say S:
S=p+q+r, where S is a prime.


Important Property of Primes:
All prime numbers except 2 are odd.
If all three distinct prime numbers p, q, and r were odd, then their sum S would be the sum of three odd numbers:
Odd+Odd+Odd=Odd.
This is possible since the sum S (which is prime) must also be odd (since the smallest possible sum of three distinct odd primes is 3+5+7=15, 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 S of two odd primes and one even prime (2) would be:
Even+Odd+Odd=Even.
Since the sum S 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 p, q, and r must be odd primes.


Analyzing Statement-I: Their sum is less than 23.
We need to find three distinct odd primes p<q<r such that their sum S=p+q+r is a prime and S<23.
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:
3+5+7=15 (15 is not prime).
The next possible combinations are:
1) 3+5+11=19 (19 is a prime number).
2) 3+7+11=21 (21 is not prime).
Any other combination of three distinct odd primes will result in a sum greater than or equal to:
3+5+13=21 (not prime), or 3+7+13=23 (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 3+5+11=19 (19 is prime).
- Set 2: {5, 7, 19} where the sum is 5+7+19=31 (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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