x + 100 + 400 = 900
x + 500 = 900
x = 900 - 500
x = 400
k tr mới tin
X + 100 + 400 = 900
X + 100 = 900 - 400
x + 100 = 500
x = 500 - 100
x = 400
X+100+400=900
X+100=900-400
X+100=500
X=500-100
X=400
k mk mk k lại cho!
X+100+400=900
=> X+ (100+400)=900
=>X+500=900
=>X = 900-500
=>X=400
x + 100 + 400 = 900
x + 100 = 900 - 400
x + 100 = 500
x = 500 -100
x = 400