You're not alone. Those of a mathematical bent have trouble, as these adverts always leave out some crucial information.
I have been in the situation and have got the T-shirt!
In order to do the pro rata calculation, we need to know the nominal full-time hours for the full-time salary.
For instance
If full-time is 40 hours, 20 hours means �7500 pa.
If full-time is 35 hours, 20 hours means �8572 pa
So without that bit of information we can't do the conversion into an hourly rate.