Q:

Relative abundance of the isotopes of iridium

There are two stable isotopes of iridium, $\ce{^191Ir}$ and $\ce{^193Ir}$. If you randomly pick an iridium atom from a large collection of iridium atoms, which isotope are you more likely to select?

A:

An urn contains nothing but

63 white balls and
37 black balls.

You grab into the urn, pick one item, look at it and put it back.
What is the probability that you drew:

a ball
a white ball
a black ball
a red ball?

