okay.

number of balls of each color
number of red balls in the left urn
number of choosers


results

number of times left urn was chosen
number of times right urn was chosen
number of times red was chosen (in right)
number of times black was chosen (in right)
number of times red was chosen (total)
number of times black was chosen (total)

So, 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..

Reminder:
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?

since this is javascript, it can take a little while to run large number of choosers, during which your browser will freeze. don't worry.

source code