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How do you find the largest number divisible by any number?

Published in Number Theory 3 mins read

The question is a bit ambiguous. Let's interpret it in a couple of ways and provide answers for each interpretation.

Interpretation 1: Largest Number Divisible by a Set of Given Numbers

This interpretation assumes we have a set of numbers, and we want to find the largest number (within a certain range, implicitly) that is divisible by all of them. A common scenario is finding the largest n-digit number divisible by a set of numbers.

Process:

  1. Find the Least Common Multiple (LCM): Calculate the LCM of the given set of numbers. The LCM is the smallest number that is divisible by each number in the set.
  2. Determine the Range (Optional): If you're looking for the largest number within a specific range (e.g., the largest 4-digit number), define the upper bound of that range. If not, you're basically looking for infinity (there is no 'largest' number, as numbers go on forever).
  3. Divide and Conquer: Divide the upper bound of the range (or a very large number if no specific range is provided) by the LCM.
  4. Multiply to Find the Largest Divisible Number: Multiply the whole number part of the result from step 3 by the LCM. This yields the largest number within the specified range (or, close to your test number) that is divisible by all the numbers in the original set.
  5. Verification: Check if the answer is correct by dividing the result by all the given numbers. All the divisions should give a whole number as the result.

Example (Based on Vedantu reference):

Find the largest 4-digit number exactly divisible by 12, 15, and 18.

  1. Find the LCM: The LCM of 12, 15, and 18 is 180. (You can find this by prime factorization or other methods).
  2. Range: We're looking for a number less than or equal to 9999 (the largest 4-digit number).
  3. Divide: 9999 / 180 = 55.55
  4. Multiply: 55 * 180 = 9900
  5. Verification: 9900/12 = 825; 9900/15 = 660; 9900/18 = 550. 9900 is indeed divisible by each.

Therefore, the largest 4-digit number divisible by 12, 15, and 18 is 9900.

Interpretation 2: Largest Number Divisible By Any Number

This is a trickier interpretation. If we interpret "any number" as meaning every possible number, then the only number divisible by every number is zero (0).

Why?

  • Any number divided by zero is undefined.
  • Zero divided by any number is zero.

If, instead, the intention is to determine whether there is a general rule applicable to any number, the most appropriate action would be to determine whether the question reflects the answer and, if needed, rephrase the query.

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