There are an unknown number of deer.
We know that there are 330 tagged deer out there.
After some time, they are presumed to be mixed into the entire herd, so if we gather in a sample, and 10% of them are tagged, we can assume that 10% of the whole herd has been tagged.
So, in a sample of 672 deer, 168 are tagged, making that 25%.
So, we can assume that the 330 deer we tagged were 25% of the whole herd, meaning that there are 1320 deer in all.
Any significant sample we gather in should be 25% tagged, just like the 672 named here. If we caught the whole herd, it would be 25% tagged, meaning the 330 we originally caught.