Cyclist M leaves point P at 12 noon and travels in a straight path at a ... ...

Cyclist M leaves point P at 12 noon and travels in a straight path at a constant velocity of 20 miles per hour. Cyclist N leaves point P at 2 PM, travels the same path at a constant velocity, and overtakes M at 4 PM. What was the average speed of N?
(A)    15
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Nova Math - Problem Set X: Word Problems