solving systems of linear equations :D
but i preffer to leave this behind my back at university also xD

A number m of the form 10x + y is divisible by 7 if and only if x − 2y is divisible by 7.

m = 1603 -> 160 - (2 x 3) -> 154             //first time
154 -> 15 - 8 = 7 and 7 is divisible by 7    //second time

Read the description again, please.

m = 1603 -> 160 - (2 x 3) -> 154 -> 15 - 8 = 7 and 7 is divisible by 7. it's in the description. 1603 is divisible be 7 too, but you need to apply the method 2 times, and that's what you should return.