The condition is equivalent to this system of 2 equations in 2 unknowns:

x - y = 235,    (1)
x + y = 885.    (2)


You can solve it in 2 (two, TWO) ways.


1.  Elimination method

    Add the two equations. Then the terms "y" will cancel each other, and you will get very simple equation for one single unknown x only:


    2x = 235 + 885  ====>  2x = 1120  ====>  x =  = 560.

    Then from eq(2)  y = 885 - x = 885 - 560 = 325.


Answer.  560 people in the second day and 325 people in the first day.



2.  Substitution method


    From eq(1) express  x = 235 + y and substitute it into the second equation. You will get

    (235+y) + y = 885,

     235 + 2y = 885  ====>  2y = 885 - 235 = 650  ====>  y =  = 325,

     and you obtain the same answer in this way, of course.