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Question

Find the four-digit dividend of 21, 42, 147 and 105 that will give the remainder 14, 35, 140 and 98, respectively.

This question was previously asked in
RRB NTPC 2024 Undergraduate CBT 1 Question Paper (29-Aug-2025) (Shift 1)
The correct answer is

8813

Let the four-digit dividend be \(x\).

According to the problem, we have the following congruences:

  • \(x \equiv 14 \pmod{21}\)
  • \(x \equiv 35 \pmod{42}\)
  • \(x \equiv 140 \pmod{147}\)
  • \(x \equiv 98 \pmod{105}\)

Notice that the remainders are related to the divisors as follows:

  • \(14 = 21 - 7\)
  • \(35 = 42 - 7\)
  • \(140 = 147 - 7\)
  • \(98 = 105 - 7\)

Therefore, we can rewrite the congruences as:

  • \(x \equiv -7 \pmod{21}\)
  • \(x \equiv -7 \pmod{42}\)
  • \(x \equiv -7 \pmod{147}\)
  • \(x \equiv -7 \pmod{105}\)

This means \(x + 7\) is divisible by 21, 42, 147, and 105. We need to find the least common multiple (LCM) of these numbers.

First, find the prime factorization of each number:

  • 21 = 3 x 7
  • 42 = 2 x 3 x 7
  • 147 = 3 x 72
  • 105 = 3 x 5 x 7

The LCM is \(2 \times 3 \times 5 \times 7^2 = 1470\).

So, \(x + 7\) is a multiple of 1470. Let \(x + 7 = 1470k\) for some integer \(k\). Then \(x = 1470k - 7\).

Since \(x\) is a four-digit number, we test values of \(k\):

  • If \(k = 1\), \(x = 1463\)
  • If \(k = 2\), \(x = 2933\)
  • If \(k = 3\), \(x = 4403\)
  • If \(k = 4\), \(x = 5873\)
  • If \(k = 5\), \(x = 7343\)
  • If \(k = 6\), \(x = 8813\)

Therefore, the four-digit dividend is 8813.

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Important Questions from Divisibility and Remainder

  1. If the 8-digit number 888x53y4 is divisible by 72, then what is the value of (7x + 2y), for the maximum value of y?

  2. If all positive divisors of 132 are arranged in descending order, then what digit will be at unit place of first divisor ?

  3. If 3 2019 is divided by 10, then what is the remainder?

  4. The number 3798125P369 is divisible by 7. What is the value of the digit P?

  5. Consider all 3-digit numbers (without repetition of digits) obtained using three non-zero digits which are multiples of 3. Let S be their sum.

    Which of the following is/are correct?

    1. S is always divisible by 74.

    2. S is always divisible by 9.

    select the correct answer using the code given below:

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