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?	Choices.
A) 16.5  B) 17.0  C) 17.5  D) 18.0  E) 18.5
time 1 = 300 / 20= 
<gadget id="calculator">300 / 20</gadget>
<output>15</output>
15 hours   time 2= 150 / 15= 
<gadget id="calculator">150 / 15</gadget>
<output>10</output>
10 hours   total time= 25 hours   average speed = 450 / 25= 
<gadget id="calculator">450 / 25</gadget>
<output>18</output>
18 km/hr   The answer is 18.0.
<result>D</result>
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?