How do I calculate average speed?

Mike is riding bicycle on a road that can be thought of as having four parts of equal length.  On the first fourth, which is level, he pedals at an average rate of 10 kilometers per hour.  On the second fourth, an upward slope, he travels at an average rate of 5 kilometers per hour.  On the third fourth, a downward slope, he goes at an average rate of 30 kilometers per hour.  On the last fourth, which is level again-but with the wind pushing him, he travels at an average rate of 15 kilometers per hour.  What is Mike's average speed over the entire road?