(1)7 | x - 2y   <=>  7 | 10x - 20y
(2)7 | 10x - 20y  <=>  7 | 10x + y

so 

7 | x - 2y  <=>  7 | 10x + y

##</code><code>##

(10 * I) % N == 1

##</code><code>##

10x + y = 5 * (10x + y) = 50x + 5y = x + 5y = x - 2y

##</code><code>##

10x + y = 19*(10x+y) = 190x +19y = x + 19x = x - 2y