Answer should be $600,$ Option A.
The problem given is equivalent to the problem in which an urn contains some number of white balls in it. We take $30$ balls out of it, mark them and put them back into the urn. Now, we randomly take $40$ balls out of the urn, $2$ of them are found to be marked. What is the approximate number of balls that were present in the urn initially?
Solution: Suppose the urn contained $X$ balls initially. Then
if we take $n$ ball out of urn, probably $n\times (30/X)$ balls will be marked out of $n$ balls.
Here, $n = 40.$
So, Probably $40*(30/X)$ out of $40$ balls will be marked.
But it is given that there are $2$ marked balls,
So, $2 = 40\times (30/X),$ which gives
$X = (40 \times 30)/2 = 600.$