If a conservationist catches 72 trout, tags them and throws them back in the lake. Later 33 trout are caught, 11 of them are tagged. How many trout would the conservationist expect to find in the lake.
1/3 of the trout turn out to be tagged.
So, 1/3 of the trout population is 72.
There must be about 216 trout there.