.
Your working equation to find the unknown width x of the uniform strip is
(23-2x)*(24-2x) = 240.
So, the question is: find two factors of the number 240 that differ in one unit.
You can find the solution MENTALLY: these factors are 15 and 16.
So, 23-2x = 15, which implies 2x = 23 - 15 = 8 and x = 4 feet.
Thus the width of the uniform strip is 4 feet.
It imlies that the dimensions of the rug are 24 - 2*4 = 16 feet (length) and 23 - 2*4 = 15 feet (width).
Solved (mentally).
You can get the same answer using other methods.