(90+2x)(40+2x)=40*90/72%
3600+80x+ 180x+4x^2=5000
4x^2+260x- 1400=0
x^2+65x-350=0
(x-5)(x+70)=0
solve
x 1=5
X2=-70 (does not meet the meaning of the question, omitted)
So, a: the width of the edge of the gold paper is 5cm.