Great idea and well executed! Had fun even though I was hopeless at it.
I had a bit of a lag issue where my calculator state kept being overridden by the server state, so I had to enter numbers slowly one at a time to make sure each keystroke was received. I wonder if it should even need to be synchronised at all until you submit a guess? On the other hand, it is pretty funny how players can try to look over each others' shoulders.