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