CAT Exam Model Paper 5 with solutions for free online practice

The first black marble should be placed between the first and the second white marble. After that a black marble should be placed after every two white marbles.
Thus the arrangement of the marbles (W = white and B = black), starting from the left-most white marble, will look like:
WBWWBWWBWWB...
In the given arrangement it can be observed that, starting from the left, for every '2k' white marbles there are 'k' black marbles. So if there are 50 white marbles, 25 black marbles are needed to ensure that each marble has atleast one neighbour of different colour.