Sushi Delight solution

Answer: The probability of removing a piranha in now quite high, much higher than 50%. The chance is 2/3 or 66.6%. Denote the piranha added to the bowl as "piranha 2." After removing the piranha from the bowl there are three equally likely states: piranha 1 in bowl, piranha 2 outside of bowl; piranha 2 in bowl, piranha 1 outside of bowl; goldfish in bowl, piranha 2 outside of bowl. In two out of three cases, a piranha remains in the bowl.

Let me clarify by adding my assumptions: When the sushi-lover reaches into a bowl containing a piranha and a goldfish, assume it is equally likely that he will withdraw a piranha as he will withdraw a goldfish. Also, it's equally likely that the bowl initially contains a piranha or a goldfish.

If you like statistics, check out the U. Florida statistics page.

If you like piranha, take a look at Piranha site.

If you like sushi, take a look at the California Sushi Academy.