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This problem is the old-fashioned (2000-3000 years old) entertainment problem.
The legend says, the solution is this:
Mr. Thom added his own horse to 17 horses of the late/deceased Mr. Klopstock.
Then he divided the 18 = 17+1 horses in this way:
1/2 of 18 = 9 to the widow;
1/3 of 18 = 6 to the elder son;
1/9 of 18 = 2 to the younger son.
9 + 6 + 2 = 17, and 18-17 = 1.
After completing his mission, Thom got on his horse and went home.
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The hidden meaning of this story is that = = = and is not 1 (one).