A farmer wants to sell sheep at a fair which lies on the other side of the hill under which he lives. In order to get there in time, he needs to travel at an average speed of 60km/h. The road is 10km up the hill and 10km down the hill. However, his car can go up the hill at maximum of 30km/h. How fast does he need to go down the hill to meet the average speed he has set?
Hint
It is not 90km/h and it is not 120km/h either.
Answer
He would need an infinite speed which means he cannot make it. (Explanation: if he needs to travel 20km at 60km/h, it will take him 20min; and the up-the-hill part of the trip is 10km at 30km/h which takes 20min, so he has no time left for the down-the-hill part of the trip)
