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.
It depends on whatever a normal working week is.15000/52=about �288 per week or �7.20 per hour, assuming a 40 hour week. Multiply that figure by hours worked to get the actual wage, in this case about �144 per week.
It`s all a bit complicated when the working week can be any other figure [eg �8.00 per hour for a 36 hour week gives a pro-rata wage of �160] but that`s the basic idea....hope it helps you in some way. Maths gives me a headache too.
If I understand you question correctly, �15k is the FULL TIME salary and you need to know what the hourly rate is. Assuming a 35hr week, then �15k equates to �8.24 per hour (15000 / 52 / 35). If you work just 20 hrs pw you will receive about �165 pw. Annually, you would recieve �15000 x 20/35 = �8571.