SOLUTION: The number of ways in which the letters x1,x2,.....,x10,y1,y2,....,y15 can be arranged in the line such that the suffixes of x and those of y are in ascending order o

1.  I don't know whether is it you the author of this text, or somebody else . . . 

    Anyway, this style asking a question, which is used in the post, is a BAD STYLE  (in my personal opinion), 

    because it demonstrates LACK of consideration / (lack of respect) from who is asking to whom is questioning.



2.  Speaking on the substance, the formulation seems to me to be inaccurate and unprofessional.

    The term "suffixes" is definitely irrelevant. What you call "suffixes", are actually "indexes".

    A professional way to formulate the problem is, for example, this


         Alex has 10 different letters of the English alphabet written in red in pieces of paper, 
         and 15 different letters of the English alphabet written in black in pieces of paper.

         Alex plays making 25-letter words comprised of these letters by placing them in line in different positions.

         How many different arrangements/words are possible, if all red letters must go in alphabetic order in their respective positions 
         and all black letters must go in alphabetic order in their respective positions, too.