Find the missing numbers ($X$ and $Y$) in the series and find the value of $Y-X$:
25, 26, 28, X, 35, Y, 46

Let's find the difference pattern between consecutive terms:
* $26-25=1$
* $28-26=2$
The difference increases by $+1$ at each step ($+1,+2,+3,+4,+5,+6$).
* $X=28+3=31$
* Check next term: $35-31=4$ (Matches pattern)
* $Y=35+5=40$
* Check next term: $46-40=6$ (Matches pattern)

Now compute $Y-X$:

$$Y-X=40-31=9$$