What if there were 24 beavers caught each time and 8 were tagged the second time?

Assuming that we are catching a random set of beavers each time, and none were tagged before we caught the first group, then there are about 72 beavers in the population. Here's why:

The second time through, 1/3 of the beavers caught were tagged. Since it's a random set from the population, then about 1/3 of the population should be tagged.

Since none of the population was tagged before we got there, and we only tagged 24, then 24 must be 1/3 of the total population.

24=(1/3)*P

solve for P:

P=72

This should get you going on your problem. If it doesn't, let me know and I'll give you another nudge.