A car travels the first kilometer of a 2km journey at an average speed of 30km/h

Question

A car travels the first kilometer of a 2km journey at an average speed of 30km/hr. At what average speed must the car travel the second kilometer in order to travel the entire journey at a average sped of 60km/hr?

Explanation / Answer

We have the distance and speed for the first leg, so we can find the elapsed time easily by using v = d/t -> time = distance/speed. So the time for the first leg is 1km/(30km/hr) or (1/30)hr We now have all the pieces we need: avg velocity = 60 = total distance / total time 60 km/hr = 2 km / (x + (1/30)hr) where x = time elapsed for second kilometer solve for x to get x = 2km/60km/hr - (1/30)hr = 0 So the car has to travel the second kilometer in 0 hrs. In other words, it cannot be done.

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