x + 400 = 500
x = 500 − 400
x = 100
 
600 + x = 900
x = 900 − 600
x = 300
 
x − 300 = 400
x = 400 + 300
x = 700
 
800 − x = 200
x = 800 − 200
x = 600
 
700 − x = 200
x = 700 − 200
x = 500
 
x − 800 = 0
x = 0 + 800
x = 800