For a new puzzle suppose that a man wishes to estimate the
number of fish in a pond.
He casts a net into the water and
pulls up 30 fish. He then marks these fish with a
tag and throws
them back. After waiting for the marked fish to be randomly
mixed into the pond population he casts the same net back into
the pond
and pulls in 40 fish of which 2 are marked. Using this
information make
an estimate of the total number of fish in the
pond.