An old farmer had 19 horses that he willed to his three children as follows: The oldest child gets half the horses, the middle child gets one fourth of the horses and the youngest child gets one fifth of the horses. When the old man died, the children did not know how to divide the horses without killing them. They asked for help from a wise neighboring farmer, who came over the next day riding on his horse. How did he solve their problem?
Solution:
The wise farmer added his horse to the 19 to make 20 horses. He gave the oldest child half of 20 horses = 10 horses; the middle child one fourth of 20 horses = 5 horses; and the youngest child one fifth of 20 horses = 4 horses. This left the wise farmer his own horse to ride home on. Why does this problem need one more horse? Because 1/2+ 1/4 + 1/5 adds up to 19/20.