can anyone help , Plz!! it's math!

Steps:
[tex]Interior +interior=exterior[/tex]
but since one of the interiors are missing we will do this.
= [tex]50+X=100[/tex]
[tex]-50[/tex] [tex]-50[/tex]
the 50 is canceled so we are left with 100-50 which is 50 so the answer is
[tex]X=50[/tex]
Inform me if its wrong.