A neighbor wanted to help so he loaned a cow to them.
Then the first son took 1/2 of 18 cows = 9 cows.
The second son took 1/3 of 18 or 6 cows.
The third son took 1/9 of 18 or 2 cows.
That makes 9 + 6 + 2 = 17 cows.   So the neighbor took his cow back home.   Explain.

Answer:
The three fractions in the father's will do not add up to one.
1/2 + 1/3 + 1/9 = 17/18.
So, the first son got more than he should have (9 cows instead of 8.5).
The second son got more than he should have (6 cows instead of 5.67).
The third son got more than he should have (2 cows instead of 1.89).
There should have been 17/18 of a cow left over...