A company is producing a one-page flysheet 24 cm high by 18 cm wide. It will have a text area in the centre which has a margin of the same width all the way around. The text must occupy exactly half of the total area:

18*24=432    432/2= 216    Therefore area of text must equal 216. And the margin must be the same width all the way round. Therefore, (24-x)(18-x)=216, 432-24x-18x+x²=216, 216-42x+x²=0, (x-36)(x-6)=0. x=36 and x=6. x cannot be 36 because the paper is not wide enough so x=6cm