Find the least perfect square that is divisible by 3, 4, 5, 6, and 8. Provide the exact number and show how it is obtained.

Aptitude Square Root and Cube Root Difficulty: Medium
Choose an option
  • A
    900
  • B
    1200
  • C
    2500
  • D
    3600

Answer

Correct Answer: 3600

Explanation

Introduction / Context: The task is to find the smallest number that is both a perfect square and a common multiple of the given set of integers. This tests understanding of LCM and perfect square conditions via prime exponents.

Given Data / Assumptions:

  • Divisibility by 3, 4, 5, 6, and 8 required.
  • Seek the least perfect square meeting these divisibility constraints.

Concept / Approach: Compute LCM of the set, then minimally adjust it to a perfect square by ensuring all prime exponents are even. Multiply by the smallest necessary factor to achieve even exponents.

Step-by-Step Solution: Prime forms: 3 = 3; 4 = 2^2; 5 = 5; 6 = 2 * 3; 8 = 2^3. LCM(3,4,5,6,8) = 2^3 * 3 * 5 = 120. To be a perfect square, each prime exponent must be even. In 120 = 2^3 * 3^1 * 5^1, exponents are odd for 2, 3, and 5. Multiply by 2 * 3 * 5 = 30 to make exponents even: 120 * 30 = 2^4 * 3^2 * 5^2 = 3600, a perfect square.

Verification / Alternative check: 3600 = 60^2 and is divisible by 3, 4, 5, 6, 8 (check quickly: 3600/8 = 450; 3600/5 = 720; etc.).

Why Other Options Are Wrong: 900 = 30^2 but 900/8 is not an integer. 1200 and 2500 are not perfect squares divisible by all listed numbers.

Common Pitfalls: Confusing LCM with the final answer without making it a square. Always ensure even exponents for primes in the factorization.

Final Answer: 3600

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