Let's say the action you're talking about is a flip of the coin, and
the "event" would be the coin coming up Heads.
For the first flip, you have a %100 chance of getting to flip, and 50%
chance it comes up heads.
There's then a 50% chance of a second flip, and a 50% chance of that
being Heads  .50 * .50 = 25%.
So...there's a 25% chance of getting to a third flip, which has a
50/50 chance of Heads  .25 * .50 = 12.5%
As you thought, the odds are 12.5%.
Cheers,,
pafalafaga 
Clarification of Answer by
pafalafaga
on
28 Dec 2006 10:22 PST
It dawned on me this might be some math homework for school, and that
you might like some extra help. If so, just let me know by posting a
note, here, and I'll provide whatever math homework help I can.
Probability is always a fascinating, but challenging topic.
Cheers,
Dave
