I'll have what she's having

Find the number of ordered pairs of integers ( x , y ) (x, y) which satisfy the equation x y = 5 x 6 y xy = 5x - 6y .


The answer is 16.

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7 solutions

The equation can be rewritten as

x y 5 x + 6 y 30 = 30 ( x + 6 ) ( y 5 ) = 30 xy - 5x + 6y - 30 = -30 \Longrightarrow (x + 6)(y - 5) = -30 .

Since 30 -30 can be factored in 16 16 ways in which the ordering of the factors matters, there will be 16 \boxed{16} corresponding ordered integer solution pairs ( x , y ) (x, y) .

@brian charlesworth Awesome transformation. I like it.

Sandeep Bhardwaj - 6 years, 7 months ago

SFFT FTW! :)

B.S.Bharath Sai Guhan - 6 years, 7 months ago

I did the exact same ¨ \ddot \smile

Akshat Sharda - 5 years, 9 months ago
U Z
Nov 7, 2014

( x + 6 ) y = 5 x (x + 6)y = 5x

y = 5 x + 30 30 x + 6 y = \frac{5x + 30 - 30}{x + 6}

y = 5 30 x + 6 y = 5 - \frac{30}{x + 6}

thus for y to be an integer 30/x+6 should be an integer

( x , y ) = { ( 36 , 6 ) , ( 21 , 7 ) , ( 16 , 8 ) , ( 12 , 10 ) , ( 11 , 11 ) , ( 9 , 15 ) , ( 8 , 20 ) , ( 7 , 35 ) , ( 5 , 25 ) , ( 4 , 10 ) , ( 3 , 5 ) , ( 1 , 1 ) , ( 0 , 0 ) , ( 4 , 2 ) , ( 9 , 3 ) , ( 24 , 4 ) } (x,y) =\{( -36 , 6) , ( -21 , 7) , ( -16 , 8) , ( -12 , 10) ,( -11, 11) , ( -9 ,15) , ( - 8 ,20) , ( -7 ,35), ( -5 , -25) , ( -4 , -10) , ( -3 , -5) , ( -1, -1) , ( 0 , 0) , ( 4 ,2) , ( 9 ,3) ,( 24 ,4)\}

thus 16 ordered pairs.

Nice solution @megh choksi

Sandeep Bhardwaj - 6 years, 7 months ago
Yash Singhal
Nov 14, 2014

Applying Simon's Favourite Factoring Trick (SFFT) in the equation, we get:

x y + 6 y 5 x 30 = 30 xy+6y-5x-30=-30 which can be factorized further as

( y 5 ) ( x + 6 ) = 30 (y-5)(x+6)=-30 .

Now, 30 -30 can be factored into 4 4 ways and there are 2 2 ways to arrange the factors among the variables x x and y y . Also, the sign can be changed in 2 2 different ways among the variables. So, in all 30 -30 can be factored into 4 × 2 × 2 = 16 4\times 2\times 2=16 ways.

Hence, there are 16 16 corresponding values of x x and y y .

So, our final answer is 16 \huge{16} .

Hamza Omar
Nov 7, 2014

xy-5x+6y-30=-30

(x-6)(y-5)=30

(x-6),(y-5)={+-((1,30),(2,15),(3,10),(5,6),(6,5),(10,3),(15,2),(30,1))}

Kristian Vasilev
Nov 9, 2014

x y = 5 x 6 y xy=5x-6y ,therefore x ( y 5 ) + 6 y = 0 x(y-5)+6y=0 ,so x ( y 5 ) + 6 ( y 5 ) = 30 x(y-5)+6(y-5)=-30 , then ( x + 6 ) ( y 5 ) = 30 (x+6)(y-5)=-30 , x and y are integers, so x+6 and y-5 are integers and the number of solutions is the number of the factors of -30, so the number of solutions is 16.

Bill Bell
Jan 26, 2015

One can obtain this result, and exhibit the collection of ordered pairs, using the Python sympy library. Use synthetic division to get that 'magic' number 30, then obtain its divisors.

Rajat De
Dec 16, 2014

A bruteforce solution will also work.

include<bits/stdc++.h>

using namespace std;

int main(){

int counter=0;

for(int i=-50;i<51;i++){

    for(int j=-50;j<51;j++){

        if(j*i==5*i-6*j){

            counter++;

            cout<<i<<" "<<j<<endl;

        }

    }

}

cout<<counter;

}

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