Arithmetic hard Q

GRE Quant Problem Solving
1

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I tried brute forcing this and chose E
3406/100 - remainder is div by 6
6812/200 - remainder is div by 6
can someone pls tell me for what values of a and b is the remainder not divisible?

2

https://gmatclub.com/forum/if-a-and-b-are-both-positive-integers-a-b-370101.html