Question 187106
(10+2x)(19+2x)=400
190+20x+38x+4x^2=400
4x^2+58x+190-400=0
4x^2+58x-210=0
(4x+70)(x-3)=0
x-3=0
x=3 in. is the ans. for the width of the border.
Proof:
(10+2*3)(19*283)=400
(10+6)(19+6)=400
16*25=400
400=400