Difficulty: Easy
Correct Answer: 91
Explanation:
Introduction / Context:
This question tests the understanding of prime and composite numbers, which is a fundamental topic in number theory and arithmetic reasoning. Composite numbers have more than two distinct positive divisors, while prime numbers have exactly two distinct positive divisors, namely 1 and the number itself. The special cases 0 and 1 are neither prime nor composite and often appear in aptitude questions to check conceptual clarity.
Given Data / Assumptions:
- The options shown are 0, 19, 29, 91, and 1.
- We use the standard definitions of prime and composite numbers for positive integers greater than 1.
- The task is to pick a number that has at least one non trivial factorization apart from 1 and itself.
Concept / Approach:
A composite number n greater than 1 has at least one divisor d such that 1 < d < n. Primes such as 19 and 29 do not have such divisors. The numbers 0 and 1 are handled separately: 1 has only one positive divisor, and 0 is divisible by every nonzero integer but is not classified as prime or composite in usual definitions. Therefore, we check each option for factorization. If a number can be expressed as a product of two smaller positive integers (both greater than 1), it is composite.
Step-by-Step Solution:
Step 1: Examine 19. It is not divisible by 2, 3, 5, or any integer smaller than its square root except 1. Therefore 19 is prime.
Step 2: Examine 29. Similar checks show that 29 is not divisible by 2, 3, 5, or any integer smaller than its square root except 1, so 29 is also prime.
Step 3: Examine 0. Zero is divisible by every nonzero integer but by convention is not considered either prime or composite, so it is not the desired answer.
Step 4: Examine 1. The integer 1 has only one positive divisor, which is itself, and therefore fails the requirement of having more than two divisors. It is neither prime nor composite.
Step 5: Examine 91. Try simple factors: 7 * 13 = 91. Both 7 and 13 are greater than 1 and less than 91, so 91 has at least two proper divisors beyond 1 and 91.
Step 6: Since 91 can be written as a product of 7 and 13, it is a composite number.
Verification / Alternative check:
To verify, list the positive divisors of 91: 1, 7, 13, and 91. Because there are more than two distinct positive divisors, 91 fits the definition of a composite number. For comparison, the divisors of 19 are just 1 and 19, and similarly for 29. The integer 1 has only the divisor 1, and 0 does not meet the standard conditions for primality or compositeness. Therefore only 91 satisfies the criterion given in the question.
Why Other Options Are Wrong:
Option 0: Although 0 is divisible by all nonzero integers, it is not classified as composite in basic number theory and is treated as a special case.
Option 19: This number has exactly two divisors, 1 and 19, and is therefore prime, not composite.
Option 29: Just like 19, the number 29 has only 1 and 29 as divisors, so it is also prime.
Option 1: The number 1 has only one positive divisor and fails the requirement for either prime or composite classification.
Common Pitfalls:
A frequent misunderstanding is to treat 1 or 0 as composite simply because they do not seem prime. Another error is not testing divisibility properly and assuming that larger numbers like 91 must be prime. Remember that checking divisibility by small primes (2, 3, 5, 7, and so on) up to the square root of the number is usually sufficient to decide primality for test level questions.
Final Answer:
The composite number among the given options is 91.
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