Problem : Explain why all radii of a circle are congruent.

A radius has one endpoint on the center and one on the circle. All points on the circle, by definition, are equidistant from the center, so no matter which point on the circle is the endpoint of a given radius, it is congruent to any other radius.

Problem : If the radius of circle P is the same length as the diameter of circle M, how long is the diameter of circle P in terms of r, the radius of circle M?

The diameter of circle P equals 4r, where r is the radius of circle M.