Question 48755
16*20=320 area of the page & (16-2x)(20-2x)=140 or 320-40x-32x+4x~2=140 or

4x~2-72x+180=0 or x~2-18x+45=0 or (x-3)(x+15)=0 or x=3 & x=-15 therefore the margin is 3 cm. 

proof (16-6)(20-6)=140 or 10*14=140 or 140=140