divisible by 7 process

divisible by 7
5 * last (y) + the rest (x)
or
the rest (x) - 2 * last (y)

10x + y
50x + 5y = (x + 5y) + 49x
49x is divisible by 7
so only need to test x + 5y

x + 5yx - 2y

augend 5 addend 2 sum 7