Let x be the total number of beavers. We mark 119 out of x beavers. Then, we found 39 marked beaver out of 89 caught beavers. So, if we build the proportion
we get, solving for x,
So, it is reasonable to suppose that there are about 272 beavers in total. Note that this is an estimate, both numerically and conceptually, since we have no information about the set of beavers caught at the beginning and the end of our experiment.