To be fully honest i am not fully educated in this and as a teenager myself i personally wouldnt have had done this at that age. However because it is phone sex and not real sex there is no chance of you getting STD or becoming pregnant so it it could be okay. on the other hand because you are only 15 and have GCSEs (depending on where you live) and exams coming up i wouldnt say it is the best use of your time.
if you both agree and enjoy , i dont see why you cant.
This is an exponential decay function of the form:
y=ab^x, In this case:
y=15000(0.8^t) so for t=3
y=15000(0.8^3)
y=$7680
The answer is 118
i hope this helps