You have to bring 4 horses from their old stable to a new one. At any given time, you can move only 2 horses by riding 1 and taking the other by the lead. Since they are all of different age, they move at vastly different speeds. The 1st horse can bridge the distance in 1 hour, the 2nd in 2 hours, the 3rd in four and the fourth in 6. How many hours does it take to move all the horses?