Prompted by the discussion here: http://news.ycombinator.com/item?id=1013789

The exchange started here: http://news.ycombinator.com/item?id=1013929

Image found here: http://zenoferox.blogspot.com/2008/02/circle-puzzle.html

Warning: spoilers.

That's barely even a puzzle. It's more of an optical illusion

Lots of people take a long time to get this one. It's what I call an STF problem - Slap the Forehead." Unless you see it instantly, when you do see it you are so frustrated ...

Argh. Yes, that's exactly how I felt when I finally got it. :p

me too. wow, it's sooooo simple. the trick is to not think.

it's more of "high school training in math has failed you"

(right triangle <=> Pythagoras) -> bad

Okay, this one is just... too easy. I can see why some people might find it confusing, though.

Without knowing that the + centers on the circle, is this actually possible?

No.

Take any circle that passes through the loweset point (under the 3" mark) and the right angle vertex that is down and left of the blue line.

Keep anything else fixed. This way infinitely many such constructions were created, and the radii were different.

No. With these sorts of problems the convention is that if it's close then it's exact, and if it was intended not to be exact then it's made obviously "off."