@ lobomattu: The solution shows that the total distance around the circuit must be 36, in order for the speed and total time criteria to be satisfied. Yet the total amount of flat distance (x) and uphill-downhill pairs (2y) remain variables, and can be satisfied by any number of (x, y) pairs---subject to the obvious constraints.

For example, let the flat terrain x = 18 km and the hills comprise the other 18 km (y = 9). With the given speeds, you'll see the cyclist makes the run in 2h 15m. Ditto for 10 km of flat terrain and 26 km of hills (13 uphill and 13 downhill), or any other x, y combo satisfying x + 2y = 36.