Let length of train is is ‘L’ km and speed ‘S’ Km/hr
L/X-11 = 20/3600 = 1/180
L/X = 9/3600 = 1/400
X = 400 L
180 L = X – 11
180L = 400L-11
220L = 11
L = 11/220 = 1/20 KM = 1000/20 =50 meters

@rajesh_balasubramanian Hello. I was just looking for an online CAT coaching and stumbled upon this. The answer seems to be 7. And i don't think 10 can be a value for B. Eg: Take the Track length as 300m. A will complete it in 50secs. And B will take 75 secs to overlap A for the first time(300/4). I can email you my solution if you'd like to give it a look.
Thank you.

@sumit-agarwal
In such scenarios like finding distinct values of [x^2/n] where x can be from 1, 2, 3 ... n[1^2/n], [2^2/n] ... [(n/2)^2/n] will yield all numbers from 0 to [n/4] (means [n/4] + 1 distinct integers)Then the next set (from [(n/2 + 1)^2/n] till [n^2/n] will be all different integers (means [n/2] distinct integers)So the number of distinct integers would be [n/2] + [n/4] + 1
if n = 100,number of distinct integers would be [100/2] + [100/4] + 1 = 76
if n = 2014,number of distinct integers would be [2014/2] + [2014/4] + 1 = 1511
if n = 13number of distinct integers would be [13/2] + [13/4] + 1 = 10
Just trying to generalize a solution shared by Kamal sir (Quant Boosters - Set 1 - Q2).
You can try out with various numbers (may be smaller numbers) so that this can be verified.

Q100) There is a 3 digit no. ABC. The no. Is a perfect square and the no. of factors of ABC is also a perfect square.
If A + B + C is also a perfect square then what is the no of factors of the six digit number ABCABC.what is the no of factors of the 6 digit no ABCABC if the cube of the product of the digits of the no ABC is not divisible by 5?