point estimation--an interview

I've encountered the following interview question:

A field with unknown number of rabbits. Catch 100 rabbits and put a label
on each of them. A few days later, catch 300 rabbits and found 60 with
labels. Estimate how many rabbits are there?


Can anyone tell me how to do this?

• capture-recapture problem which can be easy and difficult. – Deep North Sep 7 '15 at 4:45

Recall that if you have a finite population with $n_1$ subjects from group 1 and $n_2$ subjects from group 2, if you were to randomly sample a subject from the population, the probability that you get a subject from group 1 is
P(random sample from group 1) = $\frac{n_1}{n_1 + n_2}$