Modular Ratio

83 75 n ( m o d 32 ) 83 \equiv 75n \pmod{32}

What is the least possible positive integer n n satisfying the congruence above?


The answer is 25.

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2 solutions

83 75 n ( m o d 32 ) 83 \equiv 75n \pmod{32}

19 11 n ( m o d 32 ) 19 \equiv 11n \pmod{32}

Then 19 = 11 n + 32 m 19 = 11n + 32m for some integers n , m n,m .

By using Euclidean Algorithm , we can solve this linear diophantine equation:

32 = 11 2 + 10 32 = 11\cdot 2 + 10

11 = 11 1 + 10 11 = 11\cdot 1 + 10

Thus, 1 = 11 10 = 11 ( 32 11 2 ) = 3 11 32 1 = 11 - 10 = 11 - (32 - 11\cdot 2) = 3\cdot 11 - 32 .

Then 19 = 57 11 19 32 19 = 57\cdot 11 - 19\cdot 32 .

The initial solution for n n is 57 57 , and since g c d ( 11 , 32 ) = 1 gcd(11,32) = 1 , n + 32 p n + 32p will also be the solutions where p p is some integer.

Since 57 25 ( m o d 32 ) 57 \equiv 25 \pmod{32} . The least possible n n is 25 \boxed{25} .

Checking for solutions: 75 25 1875 1856 + 19 58 32 + 19 19 83 ( m o d 32 ) 75\cdot 25 \equiv 1875 \equiv 1856 + 19 \equiv 58\cdot 32 + 19 \equiv 19 \equiv 83 \pmod{32} .

very comprehensive answer.

Jia Ni Toh - 2 years, 11 months ago

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Thank you.

Worranat Pakornrat - 2 years, 11 months ago

how did you go from the first step to the second step?

Aaryan Maheshwari - 1 year, 12 months ago

If you mean literally the first two lines, then he applied mod 32 to each of the members: 83 ≡ 19 (mod 32) and 75 ≡ 11 (mod 32).

A Former Brilliant Member - 1 year, 11 months ago

6th line, the last number (last remainder) should be a 1, I suppose

Francesco Minnocci - 1 year, 10 months ago

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Oh, yes. Thank you. It's edited now.

Worranat Pakornrat - 1 year, 10 months ago
Marian Stefanescu
Nov 19, 2018

From the statement, 83 75 n 83\equiv75n (mod 32), we can derive:

{ 83 m o d 32 = r 75 m o d 32 = r { 83 = 32 k + r 75 = 32 l + r with l, r some positive integers 83 32 k = 75 n 32 l \begin{cases} 83\mod 32 = r \\ 75\mod 32 = r \end{cases} \Rightarrow \begin{cases} 83 = 32k + r \\ 75 = 32l + r \end{cases} \quad \text{with l, r some positive integers} \quad \Rightarrow \quad 83-32k=75n-32l

And thus we obtain that: 75 n + 32 ( k l ) = 83 75n + 32(k-l)=83 . We see that g c d ( 75 , 32 ) = 1 gcd(75,32)=1 . Because 1 83 1\mid 83 we can try solving the simpler problem:

75 x + 32 y = 1 x , y Z 75x^*+32y^*=1 \quad x^*, y^* \in \mathbb{Z}

Using the Euclidean algorithm we can write:

75 = 2 32 + 11 11 = 75 2 32 1 = 11 10 = 11 ( 32 2 11 ) = 3 11 32 32 = 2 11 + 10 10 = 32 2 11 = 3 ( 75 2 32 ) 32 11 = 10 1 + 1 1 = 11 10 = 3 75 + ( 7 ) 32 \begin{array}{ c|c|l} 75 = 2*32+11 & 11 = 75-2*32 & 1 = 11-10 = 11 - (32 - 2*11) = 3*11-32 \\ 32 = 2*11+10 & 10=32-2*11 & = 3*(75-2*32)-32 \\ 11 = 10*1 +1 & 1=11-10 & = 3*75+(-7)*32 \\ \end{array}

So a potential solution is: n = 3 83 = 249 , k l = 7 83 = 581 n=3*83=249, k-l=-7*83=-581 . Using the general solutions formula we get:

( 249 + 32 m , 581 75 m ) (249+32m, -581-75m)

We can see: The smallest possible n is for m = 7 n = 25 \text{The smallest possible n is for } m=-7 \Rightarrow n=25

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