Ramadhaan & Fasting

Fasts missed during one's teens

Q: I am 25 years old. I missed out fasts during my teen years and I am unsure of how many fasts I've missed. Probably around 30-40. I do however want to give kaffarah for those fasts. Should I re-estimate how many fasts I have missed and pay a kaffarah for each fast, or should I pay one kaffarah for all the missed fasts? Also, how much should one pay for a fast? Or how many people should I feed to suffice for one fast? Jazakallah in advance.