Comment by bcoates
Yeah, looking at the code my 'intuition' was "this is a classic data race, the program is broken" not "somewhere between 10 and 20".
I suppose if you assume all global accesses are atomic, it's a good demonstration that atomic operations don’t compose atomically (even in trivial cases) and aren't particularly useful as concurrency primitives.
The question is more testing your full understanding of why this is a race condition, including understanding the possible side effects, since that's the only thing you start out with when someone sends in a bug report.