This is simple direct proportion. 9kg fills 5/2 of a box (2 1/2 times the volume of the box) so 1kg fills a ninth of this which is 1/9*5/2=5/(2*9)=5/18 of a box. That means that if the volume of the box is 18 cubic units, 5/2 is 45 cubic units and will hold 9kg of noodles. So 45/9=5 cubic units will hold 1 kg of noodles.