Looking for Integers

Find the smallest positive integer n n such that the equation 455 x + 1547 y = 50 , 000 + n 455x+1547y = 50,000 + n has a solution ( x , y ) , (x,y) , where both x x and y y are integers.


The answer is 50.

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24 solutions

Sunil Pradhan
Dec 31, 2013

455x + 1547y = 50000 + n

5 × 91x + 17 × 91y = 91 × 549 + 41 + n

91(5x + 17y) = 91 × 549 + (41 + n)

(41 + n) must be divisible by 91 then 41 + n = 91

then n = 50

Nistala Rishi
May 20, 2014

First, note that 455 and 1547 are not coprime. GCD of 455 and 1547 is 91. Since x,y,n are integers, then we divide both sides by 91 to get 5x+17y = (50,000 + n)/91 = 549 + (41+n)/91 Since x,y is an integer, 549 + (41+n)/91 also has to be an integer Now, 41+n = 91k, where k is a positive integer So the minimum value of n is 91 x 1 - 41 = 50 Therefore, n is 50 To check, when n is 50, 549 + (41+n)/91 = 550 5x+17y=550 has a solution x=93, y=5 So n is 50.

This is a great way to solve the problem without reference to the Euclidean Algorithm. One must note that the fact that 50000 + n 50000+n must be divisible by 91 does not require the Euclidean Algorithm. This algorithm can be used to find a pair ( x , y ) (x,y) that gives 50050 50050 , but it can also be done by other means, especially once everything is divided by 91 91 . The Euclidean Algorithm, however, is a good way to prove the general statement, that any number divisible by G C D ( a , b ) GCD(a,b) can be expressed as a x + b y ax+by .

Calvin Lin Staff - 7 years ago
Maziar Kosarifar
May 20, 2014

The equation Ax + By = C has solution if and only if C be dividable by gcd(A, B).

By some calculation we can find that gcd(455, 1547) = 91.

So to find the smallest integer n, we can divide 50,000 by 91 and get the integer part add a one to it and multiple it by 91 so here is the number 50,050. therefore 50,050 is the smallest positive integer so n should be 50.

"The equation Ax + By = C has solution if and only if C be dividable by gcd(A, B)." This better be explained... correct, but can't feature

Calvin Lin Staff - 7 years ago

The gcd of 455 and 1547 is 91. Therefore both sides must be divisible by 91. This gives n>=50. Now for n=50, we have x=93, y=5 is a solution of the equation. Therefore the minimum possible n is 50.

Shamik Banerjee
Dec 31, 2013

n = 455x + 1547y - 50000 = 91(5x + 17y) - 50000

As 50000 = 91 * 549 + 41, the minimum possible value of n would be when (5x + 17y) is the minimum value >= 550.

5x + 17y = 550 => x = (550 - 17y)/5 = 110 - (17/5)*y This means that 5 should divide y for x to be an integer. Say, y = 5k => x = 110 - 17k.

Minimum possible integral value of n = 91 * 550 - 50000 = 50050 - 50000 = 50.

Only sensible Answer

Ritwik Sain - 7 years, 5 months ago

but x=95, y=5 has minimum ans. n=10

SHARIQ hussain - 7 years, 5 months ago
Rahul Saha
Dec 31, 2013

455 x + 1547 y = 50000 + n 455x+1547y=50000+n is a Linear Diophantine equation.A linear Diophantine equation is of the form:

a x + b y = c ax+by=c

where a a and b b are coefficients of x x and y y . We are going to use the following lemma to solve the problem at hand.

Lemma:A solution(infinite solutions,to be exact) exists if and only if g c d ( a . b ) c gcd(a.b)|c

Sketch of proof: Let g c d ( a , b ) c gcd(a,b)\nmid c . But then g c d ( a , b ) a x + b y = c gcd(a,b)|ax+by=c ,a contradiction.For more on LDE, see here

Now,back to the problem.We want the lowest possible value of n n where satisfies

455 x + 1547 y = 50000 + n 455x+1547y=50000+n

By our lemma, g c d ( 455 , 1547 ) 50000 + n 91 50000 + n gcd(455,1547)|50000+n\implies 91|50000+n

To minimize n n (which must be positive),we find the multiple of 91 91 closest to 50000[note that 50000 itself is not a multiple of 91].As n must be positive,the right hand side of the equation must be greater than 50000.Hence,we want the multiple to be greater than 50000.The closest multiple is 50050 50050 .Therefore,the following has a solution in intgers.

455 x + 1547 y = 50050 4551 x + 1547 y = 50000 + 50 n = 50 455x+1547y=50050\implies 4551x+1547y=50000+50\implies n=50

Hence the lowest possible value is 50 50 .

Kislay Raj
May 20, 2014

Given,

455x + 1547y = 50000 + n

since 455 and 1547 have the no. 7 * 13 i.e 91 in common , take out the common factor That becomes,

91(5x + 17y) = 50000 + n or , 5x + 17y = (50000 + n)91

Hence 50000 + n should be divisible by 91 to have (x,y) as integers.

the nearest multiple of 19.. to 50000 is 500050

Hence the value of n = 50

"Hence the value of n = 50" One needs to check that this works, but, since 91 is already factored out, it is easy.

Calvin Lin Staff - 7 years ago
Kiriti Mukherjee
May 20, 2014

The gcd(455,1547) = 91
So according to Euclidean Algorithm (50000+n) ≡ 0 (mod 91)
We know ,

50000 ≡ 41(mod 91)
but we have (50000+n) ≡ 0 (mod 91)
or, 41 + n ≡ 0 (mod 91)
or, n ≡ - 41 (mod 91)
or, n ≡ 91 - 41 (mod 91) or, n ≡ 50 (mod 91)
as stated the smallest positive integer . so n = 50




"So according to Euclidean Algorithm (50000+n) ≡ 0 (mod 91)" This is an inaccurate reference, the logic is unclear.

Calvin Lin Staff - 7 years ago

Since g c d ( 455 , 1547 ) = 91 gcd(455,1547) = 91 , -> 50000 + n 50000 + n must be divisible by 91 91 .

By trial and errors, we get n m i n = 50 n_{min} = \boxed{50} . ~~~

Note: 50050 = 91 × 550 50050 = 91\times550

When we get 455 x + 1547 y = 50050 455x+1547y = 50050

Factor 91 we get...

5 x + 17 y = 550 5x+17y = 550 .

Frobenius(5,17) is 63, which is less than 550. Therefore, it has a solution ( x , y ) (x,y) for this equation.

Note: Check the Frobenius number (from Chicken McNugget theory) that the number that can't be written as a x + b y ax+by .

Notenote: x = 17 n + 8 , y = 5 ( n 6 ) . x = 17n +8, y = -5(n-6). for n Z n \in Z

Notenotenote: Happy new year 2 11 34 ! ! ! 2^{11} - 34!!!

Samuraiwarm Tsunayoshi - 7 years, 5 months ago

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Frobenius numbers are those that can't be written when x, y are positive integers. You don't need to pull those out for this problem, since by Bezout's Lemma, there's a way to make 5x+17y=1, and then if we multiply each of x and y by 550, we'll make 5x+17y=550.

Michael Tang - 7 years, 5 months ago

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How do we make 5x+17y=1 to 5x+17y=550? If there exists (x,y) in 1st equation, how can we know that there also exists (x,y) in 2nd equation? (obviously exists in this one, but not always in different numbers..)

Samuraiwarm Tsunayoshi - 7 years, 5 months ago

i just did mod 91 on both sides.left side vanishes and you get n= 50 mod 91 so the answer is 50

Ashar Tafhim - 7 years, 5 months ago

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Yeah, but that's actually the same as doing this. Finding 50000 mod 91 is also ok.

Samuraiwarm Tsunayoshi - 7 years, 5 months ago
Nattapat Watanapa
Dec 31, 2013

Let y be 0, because 1547 is bigger than 455, giving a less detailed addition for each multiplication (the number adds up more per each multiplication than for 455). Then we divide 50,000 by 455, the remainder is 405. That means after we subtract 455(x-1) from 50,000, we have 405 left. The equation will be: 455 = 405 + n, so n is 50 \boxed{50}

how do u substitute y=0

Aroune AJ - 7 years, 4 months ago
Manoj Kumar
Dec 31, 2013

Given equation 455x+1547y = 50,000+n

GCD of 455 and 1547 is found to be 91.

Diving the Equation by 91 gives 5x+13y = 549+((41+n)/91) Since n has to be as small as possible, the possible least of n such that (41+n) is multiple of 91 is n = 50;

So , n= 50 is a solution if 5x+13y = 550 has a solution (x,y) with x and y as integers. Now , the above equation can be written as x+(13/5)y = 110; and so y = (110-x)*(5/13);

If y has to be an integer, (100-x) should be a multiple of 13. One such case is x =6 and y will be 40. There may me many number of cases. So, the solution exists when n =50, the least possible value in this case.

try to multiply both the numbers to reach the given nearest number.

Michael Kyllönen
May 20, 2014

Since positive integer (nonzero) was required I started by multiplying bigger number by 10, 20, 30 to find suitable starting point and then checked my already prepared list of multiples of 455.

No fine art here, just brute force.

The answer is correct, and was probably done as stated, probably with a calculator.

Calvin Lin Staff - 7 years ago
A Joshi
May 20, 2014

The second part of the expression i.e. 1547y has to be as closest as possible to 50 000 . With y = 30 , 1547y = 46410 , so the difference between 50 000 and the expression becomes 3590 . The nearest value of the first part of the expression ie 455x with x as an integer to 3590 is 3640 with x = 8 , Thus for x = 8 and y = 30 , the expression is 50050 resulting in n = 50

"he second part of the expression i.e. 1547y has to be as closest as possible to 50 000 " This is not true. In fact, in the correct answer it is not as close as possible,

Calvin Lin Staff - 7 years ago
Kalfin Muchtar
Mar 31, 2014

let y=0, then we have 455x=50000+n <=> 455 | 50000+n <=> 455 | 109*(455)+405+n so (405+n) must divisible by 455, then we get n=455-405=50.

Rusli Azis
Mar 6, 2014

455x + 1547y = 50,000 + n GCD(455,1547)=91 Then 91 I 50,000 + n.....so we get 50,000 + n = 91 x 549 + (41+n). Must be noticed that (41+n) must be divisible by 91, therefore we get 41+n=91, finally we get n the smallest positive integer is 50

Jhkhj Hjk
Jan 31, 2014

observe that 455+1547=2002 and 5000 would be 2000 25-->(2002-2) 25 --> 2*25 = 50

hope i am clear... this is easy than other strategies told..Am I wrong?

jhkhj hjk - 7 years, 4 months ago

455 * X + 1547 * Y = GCD (455, 1547) .... (1)

Greatest Common Divisor of 455 and 1547 is 91.

ceil (50000/91) = 550.

Multiplying (1) by 550 we get:

455 * 550 * X + 1547 * 550 * Y = 91 * 550 = 50050 = 50000 + 50

So n = 50.

Antonio Rangel
Jan 7, 2014

455/5 = 91

1547/91= 17

50000/91= 549.45

50000+n/91 = 550

n = 50

Ahmed Taha
Jan 6, 2014

455x+1547y=50000+n admets a non-empty set of solutions if and only if PGD(455,1547) divides 50 000+n

Which means 91 divides 50 000+n and n is minimal

hence n=50 ( first multiple of 91 that is bigger than 50 000 is 50050)

Denis Antypas
Jan 5, 2014

I don't know if this is a very good solution but it is how I solved the problem. What I did was that I tried to reach as close as possible to 50,000 multiplying 1547 with a possible value of y. Then, I multiplyied 455 with a possible value of x so that when added to 1547*y would exceed 50,000 by the minimum number possible. What I did was this:

I set y=30: 1547*30=46,410.

46,410 is close to 50,000.

I set x=8: 455*8= 3640.

46,410+ 3,640=50.050

So, 455 x + 1547 y = 50,000 + n <=> 50,050 = 50,050 + n <=> 50,050 - 50,000 = n <=> n = 50

Budi Utomo
Dec 31, 2013

455 x 110 = 50050 ---> 50050 - 50000 = 50

Krish Shah
Apr 14, 2020

Using the Euclidean Algorithm with ( 1547 , 455 ) (1547, 455) , we get

1547 = 455 ( 3 ) + 182 1547 = 455(3) + 182

455 = 182 ( 2 ) + 91 455 = 182(2) + 91

182 = 91 ( 2 ) + 0 182 = 91(2) + 0

So, we know that g c d ( 1547 , 455 ) = 91 gcd(1547, 455) = 91

Now, going backwards, we see that (I've skipped the steps, refer to Extended Euclidean Algorithm )

91 = 455 ( 7 ) 2 ( 1547 ) 91 = 455(7) - 2(1547)

As 50 , 000 / 91 = 550 ⌈50,000/91⌉ = 550 , we can multiply 91 91 with 550 550 to get 50 , 050 50,050 .

Finally,

50 , 050 = 50 , 000 + n 50,050 = 50,000 + n , yielding n = 50 n = 50 .

Mohd. Hamza
Nov 7, 2018

Since the least possible value of 455 x + 1547 y 455x + 1547y where ( x , y ) (x,y) are integers is 91 91 and according to Bezout's indentity the solution to this equation is every multiple of 91 91 .

And 50 , 000 / 91 50,000/91 is approximately equal to 549 549 .

Therefore 50 , 000 + n = 550 × 91 = 50 , 000 + 50 50,000 + n = 550×91 = 50,000 + 50 Hence the answer is 50 50 .

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