CAT 2011 question

Source: Of course CAT 2011

Problem:    Suppose you have a pond and in that pond you have some goldfish. Size of all the fish is same and they are evenly distributed in the pond.

You want to count them. You threw the net inside the pond and saw that there are 40 fish in the net. You marked all of them with blue color.

You repeated the same activity and this time you got 60 fish out of them 4 had the blue color mark on them(it means they were from the first withdraw).

What is the approximate number of fish in the pond?

a.      200
b.      400
c.       600
d.      800