On a trip covering 450 km, a bicyclist travels the first 300 km at 20 km per hour and the remainder of the distance at 15 km per hour. What is the average speed, in km per hour, for the entire trip?	Choose the correct choice from the following answers:	A) 16.5	B) 17.0	C) 17.5	D) 18.0	E) 18.5

Rationale: "time 1 = 300 / 20 = 15 hours time 2 = 150 / 15 = 10 hours total time = 25 hours average speed = 450 / 25 = 18 km / hr the answer is d ."
Answer: D