Answer:  There are 467 students at Holmes Junior High.
               There are 342 students at Harper Junior High.
_________________________________________________
Step-by-step explanation:
_________________________________________________
Let " x" represent the number of students at Harper Jr. High.
Holmes Junior has:  " (x + 125) "  students .
Harper Junior High has:  " x"  students:
_________________________________________________
Together, the total number of students equal:  " 809 " students.
so:  " x + (x + 125) = 809 " .
We want to solve for "x" and " (x + 125)" .
We want to solve for "x" ;  
 then we can solve for " (x + 125) "  by adding "125" ;
                                                       to our "solved value for "x" ;
____________________________________________
   x + (x + 125)  = 809 .
  →  1x + 1x + 125 = 809 ;
  →   2x + 125 = 809  ;
Now, subtract "125" from each side of the equation:
  →   2x + 125 - 125 = 809 - 125 ;
  →   2x  =  684 ;
Now, divide each side of the equation by "2" ;
     to isolate:  "x" on one side of the equation; & to solve for "x" ;
  →   2x/ 2 =  684 / 2  ;
to get:
  →    x = 342 ;  which is the total number of students at Harper Jr. High.
Now,  to solve for:  " (x + 125)" ;
Plug in:  "342" for "x" ;
 →  " (342 + 125) " = " 467 " '
 →  which is the total number of students at Holmes Jr. High.
_________________________________________________
Do our answers make sense:
Does:  " 342 + 467 =?  809 ?? " ;
          →  Yes!
_________________________________________________
Answer:  There are 467 students at Holmes Junior High.
                There are 342 students at Harper Junior High.
_________________________________________________