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Question

Find the LCM of 72, 78, and 90.

This question was previously asked in
RRB ALP 2025 CBT 2 Heat Engine Question Paper (28-Jul-2026) (Shift 2)
The correct answer is

4680

To find the Least Common Multiple (LCM) of 72, 78, and 90, we can use the method of prime factorization.

  1. First, let's find the prime factorization of each number:
    • 72: \(72 = 2^3 \times 3^2\)
    • 78: \(78 = 2^1 \times 3^1 \times 13^1\)
    • 90: \(90 = 2^1 \times 3^2 \times 5^1\)
  2. Identify the highest power of each prime number present in the factorizations:
    • \(2\): Maximum power is \(2^3\) (from 72).
    • \(3\): Maximum power is \(3^2\) (from 72 and 90).
    • \(5\): Maximum power is \(5^1\) (from 90).
    • \(13\): Maximum power is \(13^1\) (from 78).
  3. Multiply these highest powers to get the LCM: \(LCM = 2^3 \times 3^2 \times 5^1 \times 13^1\).
  4. Calculate the product:
    • \(2^3 = 8\)
    • \(3^2 = 9\)
    • \(5^1 = 5\)
    • \(13^1 = 13\)

Now, we simplify the calculations step by step:

  • \(8 \times 9 = 72\)
  • \(72 \times 5 = 360\)
  • \(360 \times 13 = 4680\)

Therefore, the LCM of 72, 78, and 90 is 4680.

Hence, the correct answer is 4680.

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