UPSC Prelims 2023·CSAT·Quantitative Aptitude·Number System

Consider the following in respect of prime number p and composite number c. 1. p + c{p - c} can be even. 2. 2p + c can be odd. 3. pc can be odd. Which of the statements given above are correct?

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Last updated 23 May 2026, 3:31 pm IST
  1. A1 and 2 only
  2. B2 and 3 only
  3. C1 and 3 only
  4. D1, 2 and 3Correct

Explanation

To determine why all three statements are correct, we test each with simple examples of prime numbers (p) and composite numbers (c). Statement 1: p + c divided by p - c can be even. If we choose p = 13 and c = 9, then p + c is 22 and p - c is 4. This does not result in an integer. However, if we choose p = 17 and c = 15, then p + c is 32 and p - c is 2. Dividing 32 by 2 gives 16, which is an even number. Thus, statement 1 is correct. Statement 2: 2p + c can be odd. The term 2p is always even because any number multiplied by 2 is even. For the sum to be odd, c must be an odd composite number. If we choose p = 3 and c = 9, then 2(3) + 9 equals 15, which is odd. Thus, statement 2 is correct. Statement 3: pc can be odd. The product of two numbers is odd only if both numbers are odd. If we choose an odd prime like p = 5 and an odd composite like c = 9, their product is 45, which is odd. Thus, statement 3 is correct. Since all three statements can be true under specific conditions, the correct answer is D (1, 2, and 3).
Quantitative Aptitude: Consider the following in respect of prime number p and composite number c. 1. p + c{p - c} can be even. 2. 2p + c can b

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