x+700=900-100
x+700=800
x = 800-700
x = 100
x+300=400+500
x+300=900
x = 900-300
x = 600
x+100=900+300
x+100= 1200
x = 1200 - 100
x = 1100
x+700=900-100
x+700=800
x =800-700
x =100
x+300=400+500
x+300=900
x =900-300
x =600
x+100=900+300
x+100=1200
x =1200-100
x =1100
x + 700 = 900 - 100
x + 700 = 800
x = 800 - 700
x = 100
x + 300 = 400 + 500
x + 300 = 900
x = 900 - 300
x = 600
x + 100 = 900 + 300
x + 100 = 1200
x = 1200 - 100
x = 1100