It`s either 100,000 because there is only four 0`s making it 70,000 or it`s 700,000 and you made a mistake of not typing that last zero in which case the question would be pointless to ask because the answer is in the question.
True.
Since

,

and the solutions are decreasing functions.
30 / 500 = x / 900....$ 30 tax / 500 item = $ x tax on 900 item
cross multiply
(500)(x) = (900)(30)
500x = 27000
x = 27000/500
x = 54 <===