Well, if you can visualise the two oscillating pendulum, how are you measuring 1 oscillation?
It can be taken, in this problem, as when the bob starts from one maximum, goes to the opposite maximum and back to the initial maximum.
In your problem, Y is at the opposite maximum.
Can you deduce the phase difference?
Or... let's say if you were to release the two bobs so that they oscillate as in the problem, but starting from the same point. You would obviously hold both at the same maximum point, let Y go first, and when Y reaches the other maximum, you let go X. How much of a cycle did Y go through before X is released?