A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 900 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?  Choose the correct choice:
A) 671.5  B) 600  C) 672.5  D) 673  E) 773.5
Many approaches are possible, one of them:   Let the distance be 1 kilometer.   Time to cover this distance at 900 kilometers per hour is 1/900 hours = 3,600/900 seconds = 4 seconds;   Time to cover this distance at regular speed is 900+2= 
<gadget id="calculator">900+2</gadget>
<output>902</output>
902 seconds= 902/3,600 hours = 1/600 hours;   So, we get that to cover 1 kilometer 1/600 hours is needed --&gt; regular speed 600 kilometers per hour (rate is a reciprocal of time or rate=distance/time).   Answer: 600.
<result>B</result>
A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 900 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?