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Find the number of solutions in positive integers of the equation 3 x + 5 y = 1008 3x+5y=1008 .


The answer is 67.

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3 solutions

We have 3 x + 5 y = 1008 3 ( x 1 ) + 5 y = 1005 3x+5y=1008 \Leftrightarrow 3(x-1)+5y=1005 .

Since 5 x 1005 0 ( m o d 5 ) 5x\equiv1005\equiv0\pmod{5} , we get 3 ( x 1 ) 0 ( m o d 5 ) 3(x-1)\equiv0\pmod{5} or x 1 ( m o d 5 ) x\equiv 1\pmod{5} .

Let x = 5 k + 1 x=5k+1 , where k k is an integer.

Thus, y = 1005 3 ( x 1 ) 5 = 1005 15 k 5 = 201 3 k y=\dfrac{1005-3(x-1)}{5}=\dfrac{1005-15k}{5}=201-3k .

Because x , y > 0 x,y>0 , we have: { 5 k + 1 > 0 201 3 k > 0 0 x 66 \left\{\begin{array}{l}5k+1>0\\201-3k>0\end{array}\right. \Leftrightarrow 0\le x\le 66 .

So, the equation has 67 \boxed{67} positive integer solutions.

Paola Ramírez
Aug 9, 2015

x = 336 5 y 3 3 y x=336-\frac{5y}{3}\Rightarrow 3|y .

Also x , y > 0 x,y>0 , so, 0 < 336 5 y 3 5 y 3 < 336 0 < y < 201 0<336-\frac{5y}{3} \Rightarrow \frac{5y}{3} <336 \Rightarrow 0<y<201 .

The number of solutions is 201 3 = 67 \frac{201}{3}=\boxed{67} which includes all y y divisible by 3 3 , greatest than zero but less than 201.

Tom Engelsman
Jan 31, 2017

Solving for y y in terms x x produces: y = 1008 3 x 5 = 3 ( 336 x ) 5 . y = \frac{1008 - 3x}{5} = \frac{3(336 - x)}{5}. In order for y y to be a positive integer, we must have 5 ( 336 x ) 5|(336 - x) . This yields x = 1 , 6 , 11 , 16 , . . . , 326 , 331 x = 5 k 4 ; k = 1 , 2 , . . . , 67. x = {1, 6, 11, 16,..., 326, 331} \Rightarrow x = 5k - 4; k = 1, 2,..., 67. Thus there are 67 \boxed{67} ordered-pairs ( x , y ) ; x , y N . (x, y); x, y \in \mathbb{N}.

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