Solution for Arab Horses ...

The solution is that friend lent them another horse, to make a total of 18 horses.

Then the oldest son could have half of the horses i.e. 9 horses

The middle son could have a third of the horses i.e. 6 horses

The youngest son could have a ninth of the horses i.e. 2 horses

This gives a total of 17 horses to the sons, and the friend took his own horse away.