with a little (sober) thought it was obvious that you were right,
but by the time i was halfway thru writing this code i was still not quite sober..
Two urns, one on the left, one on the right.
A bunch of red & balck marbles, same number of each.
All the marbles go into the urn on the right, except for a certain number of red marbles,
which are put in the urn on the left.
Somebody comes along and chooses an urn and random
and then takes a ball outta it.
What is the probability they'll choose a red ball?
during which your browser will freeze. don't worry.