Painfully Easy

I flip a penny and a dime and hide the result from you. “one of the coins came up heads”, i announce. what is the chance that the other coin also came up heads?

Solution

Assuming complete honesty on the part of the flipper, wouldn’t the solution be 33%?

There are four possible scenarios:

HH
TH
HT
TT

Obviously the TT possibility can be discounted because it does not result in one of the two being heads.

This lees us with three possibilities, only one of which has the other coin also being heads. 

Therefore one third.

I think.

I usually get these wrong.