Alright, here's the odds for,
>Finding an Abra at all = 30% of 100 = 30
Finding Abra with Synchronize = 50% of 30 = 15
Finding Abra with Synchronize and Modest nature = 50% of 15 = 7.5% chance
Without the odds of finding Abra though, the chances are:
>With Synchronize = 50% of 100 = 50
With Synchronize and Modest nature = 50% of 50 = 25% chance
Reasoning behind calculations: Firstly, the chance rate of finding an Abra in the Safari Zone is 30% (source) so I took a full percentage 100 and took a 30 percent out of it. Without that, we are left assuming that all your encounters are Abras (meaning a 100% chance, which means it can be ignored). This was in case you were hunting only for Abra.
Abra has two possible abilities, hence the chances of having any one ability (Synchronize here) is 50%. And lastly, the chance of Synchronize working and passing Modest nature is also 50%.
Hope I helped!