Properties of expressions with prime and composite numbers
Question
Consider the following in respect of prime number p and composite number c.
Which of the statements given above are correct?
Options
1 and 2 only
2 and 3 only
1 and 3 only
1, 2 and 3
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.
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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