a = b + 200
c = b + 400
d = 800 + c
Can't be solved, at least not to where you get an exact number. There are 4 variables (a, b, c, d) but only 3 equations.
You can get an answer, but it will always have a letter (a, b, c, or d) in it.
Is that the complete problem? Is there anything else in the problem that you might not have put in this post?