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Q15·CSAT · Prelims 2023

Properties of expressions with prime and composite numbers

NumericalPrimes & CompositesNumber theoryMedium

Question

Consider the following in respect of prime number p and composite number c.

1(p + c) / (p - c) can be even.
22p + c can be odd.
3pc can be odd.

Which of the statements given above are correct?

Options

a

1 and 2 only

b

2 and 3 only

c

1 and 3 only

d

1, 2 and 3

Answer

Explanation

Since the statements use the permissive modular helper "can be", finding a single valid numerical example validates each statement[cite: 3136, 3139, 3145].

Statement 1: Let p = 5 (prime) and c = 9 (composite). p+c/p-c = 5+9/5-9 = 14/-4, not an integer. Let's try p = 13, c = 9 \rightarrow 13+9/13-9 = 22/4. Let's try p = 7, c = 9 \rightarrow 16/-2 = -8 (even integer). Thus, it can be even. Statement 1 is correct.

Statement 2: 2p + c. Let p = 3 and c = 9. 2(3) + 9 = 6 + 9 = 15 (odd). Thus, it can be odd. Statement 2 is correct.

Statement 3: pc. Let p = 3 and c = 9. 3 × 9 = 27 (odd). Thus, it can be odd. Statement 3 is correct.

All three scenarios are mathematically possible.

For existence proofs ("can be"), quickly scan the boundary properties of low odd composites (like 9 or 15) to confirm validity immediately.

Answer: (d).

Question details

Year

2023

Paper

CSAT

Question

Q15

Section

Numerical Ability

Sub-topic

Primes & Composites

Type

Number theory

Difficulty

Medium

Source hint

Number theory

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