In how many ways can $20$ letters $x_1,x_2,\ldots ,x_{10},y_1,y_2, \ldots ,y_{10}$ be arranged in a line so that the suffixes of the letters $x$ and also those of the letters $y$ are respectively in ascending order of magnitude?
My Solution:
If both $x$ and $y$ have to be grouped accordingly, then this would be quite simple. But I don't think it's that simple.
From what I can deduce from this question, Any of the X can alternate with the Y as long as they're in ascending order respectively.
Can anyone guide me on what to do?