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Question

Find the least number which when divided by 4, 6, 8, 10 and 12 leaves the remainder 3 in all cases.

The correct answer is
123

The problem asks for the least number that leaves a remainder of 3 when divided by 4, 6, 8, 10, and 12.

Finding the Least Common Multiple (LCM)

To find the least number satisfying the condition, we first need to find the Least Common Multiple (LCM) of the divisors: 4, 6, 8, 10, and 12.

  1. Prime Factorization: Find the prime factors of each number.
    • $4 = 2^2$
    • $6 = 2 \times 3$
    • $8 = 2^3$
    • $10 = 2 \times 5$
    • $12 = 2^2 \times 3$
  2. Calculate LCM: The LCM is found by taking the highest power of each prime factor present in any of the numbers.
    • The prime factors involved are 2, 3, and 5.
    • Highest power of 2 is $2^3$.
    • Highest power of 3 is $3^1$.
    • Highest power of 5 is $5^1$.

    Therefore, $\text{LCM}(4, 6, 8, 10, 12) = 2^3 \times 3^1 \times 5^1 = 8 \times 3 \times 5 = 120$.

Calculating the Final Number

The problem states that the number leaves a remainder of 3 in all cases. This means the required number is 3 more than the LCM of the divisors.

Least Number = LCM + Remainder

Least Number = $120 + 3 = 123$.

Verification

Let's check if 123 leaves a remainder of 3:

  • $123 \div 4 = 30$ remainder $3$ ($123 = 4 \times 30 + 3$)
  • $123 \div 6 = 20$ remainder $3$ ($123 = 6 \times 20 + 3$)
  • $123 \div 8 = 15$ remainder $3$ ($123 = 8 \times 15 + 3$)
  • $123 \div 10 = 12$ remainder $3$ ($123 = 10 \times 12 + 3$)
  • $123 \div 12 = 10$ remainder $3$ ($123 = 12 \times 10 + 3$)

The condition holds true for the number 123.

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Important Questions from LCM and HCF

  1. The HCF and LCM of two numbers are 12 and 72, respectively. If the ratio of the two numbers is 2 ∶ 3, then the larger of the two numbers is:

  2. Find the greatest number that will divide 43, 91 and 183 so as to leave the same remainder in each case.

  3. Joseph visits the club on every 5 th day, Harsh visits on every 24 th day, while Sumit visits on every 9 th day. If all three of them met at the club on a Sunday, then on which day will all three of them meet again?

  4. What is the least number which when divided by 12,20 and 24 leaves in each case a remainder of 8?

  5. Which of the following is a pair of co-primes?

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