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There is such a positive integer: if it is divided by 5, the remainder is 3; if it is divided by 6, the remainder is 4; if it is divided by 7, the remainder is 1. Find the least possible value of such number.

Respuesta :

To solve this problem, we can use the Chinese Remainder Theorem.

According to the Chinese Remainder Theorem, if we have a system of congruences of the form:

x ≡ a1 (mod m1)

x ≡ a2 (mod m2)

...

x ≡ an (mod mn)

where the mi are pairwise coprime (that is, they have no common factors other than 1), then there exists a unique solution for x (modulo the product of the mi) that satisfies all of the congruences.

In this case, we have the following system of congruences:

x ≡ 3 (mod 5)

x ≡ 4 (mod 6)

x ≡ 1 (mod 7)

Since 5, 6, and 7 are pairwise coprime, we can use the Chinese Remainder Theorem to find a solution for x.

The least possible value of x that satisfies all of the congruences is the one that is congruent to 0 (modulo the product of the mi). In this case, the product of the mi is 5 * 6 * 7 = 210, so we want to find the least value of x that is congruent to 0 (modulo 210).

We can solve for x using the following steps:

Find the inverse of 5 (modulo 6). The inverse of 5 (modulo 6) is 5 because 5 * 5 ≡ 1 (mod 6).

Multiply the inverse from step 1 by 3 (modulo 6) to get x ≡ 15 (modulo 30).

Find the inverse of 6 (modulo 7). The inverse of 6 (modulo 7) is 5 because 6 * 5 ≡ 1 (mod 7).

Multiply the inverse from step 3 by 4 (modulo 7) to get x ≡ 20 (modulo 42).

Find the inverse of 7 (modulo 5). The inverse of 7 (modulo 5) is 3 because 7 * 3 ≡ 1 (mod 5).

Multiply the inverse from step 5 by 1 (modulo 5) to get x ≡ 3 (modulo 35).

Find a solution for x that is congruent to 0 (modulo 210) using the values from steps 2, 4, and 6. We can do this by adding 210 to each of the values until we get a value that is congruent to 0 (modulo 210).

For example, starting with the value from step 2 (15), we can add 210 to get x ≡ 225 (modulo 210).

Starting with the value from step 4 (20), we can add 210 to get x ≡ 230 (modulo 210).

Starting with the value from step 6 (3), we can add 210 to get x ≡ 213 (modulo 210).

Since all three of these values are congruent to 0 (modulo 210), they are all valid solutions for x. The least of these values is 225, so the least possible value of x that satisfies all of the congruences is 225.

Therefore, the answer to the problem is 225.

Answer:

  • The least possible number is 148

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Let the number be N.

If N is divided by 5 the remainder is 3:

  • N = 5k + 3,

Similarly:

  • N = 6p + 4,
  • N = 7q + 1.

We can observe N + 2 is divisible by both 5 and 6, then it can be represented as:

  • N + 2 = 30m,

so

  • 30m = 7q + 1 + 2 or
  • 30m - 3 = 7q

The left side is divisible by 3, then q = 3t:

  • 30m - 3 = 7*3t
  • 10m - 1 = 7t

The least m and t would be m = 5 and t = 7.

Then:

  • N + 2 = 30*5 = 150 ⇒ N = 148 is the least possible number.

Proof:

  • 148/5 = 29 (rem 3)
  • 148/6 = 24 (rem 4)
  • 148/7 = 21 (rem 1)

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