Question 696696
Solution :

It is given that the speed of A is 4 rotations / 120 second 
This is equal to 1 rotation / 30 sec

Similarly , the speed of B is 3 rotations / 60 sec
or 1 rotation / 20 second

Now, let us take the HCF of the time taken by both the balls A and B.
i.e. HCF of 30 and 20 is 60.

Therefore , the balls meet at the starting point at every 60 mins interval.