440 as it is the number that is halfway between them, or half of their total.
420, 450, 480
400
420, 480, 540
240 is half-way in between 0 and 480.
500
480 of them, of course! dividing by 10 drop one zero off the number being divided
my answer would be 400 + 80 = 480
25,30
400 + 20% is 400*1.20 = 480
480-400 b.C.
400x1.20=480