Question 26465
The area is given by (x + 30) (x + 40)  = 1800

x^2  + 70x  + 1200 =  1800

x^2 + 70x  -600   =   0


Next we use the quadratic equation. 

-70 +,-  Square root 4900 + 2400  /   2 

= 7.720  (disregard the negative answer)

The areas are 37.720 *  47.720, which resolves to roughly ~1800 as a check.

Therefore, the width of the walkway is ~ 7.720


Cleomenius.