The father says to divide his camel herd 1/2 + 1/3 + 1/9
Placing over a common denominator = 9/18 + 6/18 + 2/18 = 17/18 (notice there is a difference of 1 between the numerator and the denominator)
If there are 17 camels, adding 1 camel allows the eldest to have 9 camels, the 2nd son 6 camels and the 3rd son 2 camels, leaving the 1 remaining camel initially added.
The father had a herd of 17 camels.