The shares of horses add to
.. 1/2 +1/3 +1/9 = 9/18 +6/18 +2/18 = 17/18
If 1 horse is added, so there are 18 horses, then they can be divided according to the farmer's will and 1 horse will be left over.
The oldest daughter gets 9 horses.
The middle daughter gets 6 horses.
The youngest daughter gets 2 horses.
