Fog Creek Software
Discussion Board

Painfully easy

The guy that solved this riddle is just kidding, right? The question is: "what is the chance that the OTHER coin also came up heads?". The chance that the OTHER coin also came up heads is exactly 50%: tails or heads... :) and it has nothing to do with the state of first coin.

Thursday, March 7, 2002


Michael H. Pryor
Thursday, March 7, 2002

For the love of all that is good and holy, not this thread again!

<run away screaming>

Chad Hulbert
Friday, March 8, 2002

Heh.  Heh heh heh.

Paul Brinkley
Friday, March 8, 2002

Here's a new outlook (I think -- I couldn't force myself all the way thru the last discussion of probabilities and odds and the superficial distinctions therein).  In the statement, "One of them is heads" no distinction is made between the dime and the penny.  Therefore, HT and TH are actually the same event, and are contrasted only with HH, leaving a .50 probability.  Which also satisfies the obvious feeling that of course there's a 50/50 chance of a heads.

Any further input?

Saturday, April 13, 2002

Both coins were already tossed and the results can't change. We know, in advacne, that TT didn't happen.

So if it's HH the "probability" is 1. In case of TH probability is 0 and the same with HT. All the three possibilities support the statement that one of them is H.

But HH per se has 1/3 probability of occurrence, the same for the others. Then: 1/3 * ( 1 + 0 + 0) = 1/3.

If we were tossing one coin, observing it was a H and then tossing the other one of course it would be .5, as the TH case would also be ruled out.

More details in the long thread linked above.

Saturday, April 13, 2002

*  Recent Topics

*  Fog Creek Home