### The annuity value of additional payments, PV, payment amounts

I just bought a 10 year ordinary annuity that promises to pay 1,500 at the end of each next 10 years. If the appropriate interest rate for this annuity is 7.8% what is the present value of this annuity? If I want to extend my annuity another ten years, if I pay more money today, I can continue to receive $1,500 per year for