@quid: Why does it seem that people fight to keep PSQs like
this? There's nothing even really valuable here; either use symmetry or count carefully, and the asker could easily have done the latter (but didn't because of course Math SE is well-known for giving full answers to PSQs). Does the pretty picture really make the thread valuable? (I'm not sarcastic here, I do think it's pretty but squandered on a poor question.)