No problem
[QUOTE=Chronos]
I would phrase the reason as “you’re subtracting off a multiple of 21, and then dividing by ten”.
[/QUOTE]
That’s how I would have explained it.
You can construct similar tests for divisibility by 13 and 17, by the way.
In over 50 years of playing, teaching and reading about chess, I have never seen QG used as an abbreviation for Queen’s Gambit.
(In chess openings classification, it would be ECO D2 + D3.)
NIC uses QG for the QGA, and QO for the QGD.