Bob78164 wrote:

I'm pretty sure I've figured out this week's Riddler Express. The Riddler Classic appears to be more of a challenge.

The Riddler Express: Fix a circle. Choose three points at random (uniform distribution) on the circle. (To be clear, that's on the circle itself, not in its interior.) Those three points form a triangle. What is the probability that the center of the circle lies within the triangle? My answer: 0.25.

The Riddler Classic: Fix a sphere. Choose four points at random (uniform distribution) on the sphere. (To be clear, that's on the sphere itself, not in its interior.) Those four points form a tetrahedron. What is the probability that the center of the sphere lies within the tetrahedron? I don't think the technique I used to figure out the Riddler Express generalizes to this case. --Bob

I don't think the uniform distribution constraint applies to the Express puzzle. Otherwise, I think the probability would be 1.00.