Find the number of solutions in positive integers of the equation 3 x + 5 y = 1 0 0 8 .
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x = 3 3 6 − 3 5 y ⇒ 3 ∣ y .
Also x , y > 0 , so, 0 < 3 3 6 − 3 5 y ⇒ 3 5 y < 3 3 6 ⇒ 0 < y < 2 0 1 .
The number of solutions is 3 2 0 1 = 6 7 which includes all y divisible by 3 , greatest than zero but less than 201.
Solving for y in terms x produces: y = 5 1 0 0 8 − 3 x = 5 3 ( 3 3 6 − x ) . In order for y to be a positive integer, we must have 5 ∣ ( 3 3 6 − x ) . This yields x = 1 , 6 , 1 1 , 1 6 , . . . , 3 2 6 , 3 3 1 ⇒ x = 5 k − 4 ; k = 1 , 2 , . . . , 6 7 . Thus there are 6 7 ordered-pairs ( x , y ) ; x , y ∈ N .
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We have 3 x + 5 y = 1 0 0 8 ⇔ 3 ( x − 1 ) + 5 y = 1 0 0 5 .
Since 5 x ≡ 1 0 0 5 ≡ 0 ( m o d 5 ) , we get 3 ( x − 1 ) ≡ 0 ( m o d 5 ) or x ≡ 1 ( m o d 5 ) .
Let x = 5 k + 1 , where k is an integer.
Thus, y = 5 1 0 0 5 − 3 ( x − 1 ) = 5 1 0 0 5 − 1 5 k = 2 0 1 − 3 k .
Because x , y > 0 , we have: { 5 k + 1 > 0 2 0 1 − 3 k > 0 ⇔ 0 ≤ x ≤ 6 6 .
So, the equation has 6 7 positive integer solutions.