Question 250431
20*40+660=(20+x)(40+x)
800+660=800+60x+x^2
x^2+60x-660=0
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
x=(-60+_sqrt[60^2-4*1*-660])/2*1
x=(-60+-sqrt[3600+2640])/2
x=(-60+-sqrt6240)/2
x=(-60+-78.99)/2
x=(-60+78.99)/2
x=19.99/2
x=9.497 ans. for the width of the cement walk.
Proof:
20*40+660=(20+9.497)(40+9.497)
800+660=29.497*49.497
1460~1460