I got this question and I cannot event understand what should I find.

You are a shepard, and you buy regularly 20 sheeps every day. You set them free and on average, sheeps come back to your ranch ten distinct (but not necessarily consecutive) days before being eaten by a wolf. How many sheeps do you have in your ranch everyday?

What I can say is that I'm a stupid farmer..
I think the right answer is a range, I can have only the new sheeps in my ranch(20), but how can I find the maximum if I don't know the total amount of sheeps? Or is totally wrong and I can calcultate the exact amount of sheeps?

Thanks