Let's say that social security promises you $48,000 per year (paid annually) starting when you retire 45 years from today (the first $36,000 will...

Let's say that social security promises you $48,000 per year (paid annually) starting when you retire 45 years from today (the first $36,000 will come 45 years from now). If your discount rate is 8%, compounded annually, and you plan to live for 20 years after retiring ( so that you will get a total of 20 payments), what is the value today of social security's promise?

I'm confused about this question, especially the $36,000 part.

I'm using the formula PV= C/(1+r)^n

r= 8%, n=20