How fast should you drive towards a red traffic light to see it appear as green?

What is the mean distance between two randomly choosen points within a sphere of radius 1?

Show that there exist only five platonic solids (that are objects which are build out of the one sort of regular polygons, like the tetrahedron and the cube). Hint: use that for each solid in three dimensions it holds that the number of vertices + the number of faces is equal to the number of edges + 2.