Solution: Let the width of gold paper edge be x cm, and the equation can be listed.

(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.