solve this word problem?

Question

one of the way that biologists estimate the number of individuals of a species in an area is as follows. first, 100 animals of the species are caught and marked. a period of time is permitted to elapse, and then b animals are caught. if c of these(c is less than or equal to b) are marked, find the number of individuals in the area.

Answer 1

Its a proportion:
c marked/100 marked = b found/ total in area.
This assumes the unmarked animals didn't boot the marked ones out to screw up the biologists!!