Hello,
Say the area of the lawn is $\displaystyle A$, and the area of the path is $\displaystyle A'$. We are given the dimensions of the lawn, so $\displaystyle A = 30 \times 40 = 1200$.
Now you know that $\displaystyle A' = \frac{A}{4} = 300$.
Let us define a function that gives the area of the concrete path with respect to some width, denoted $\displaystyle x$. After investigation, we find that this function is defined by : $\displaystyle f(x) = 60x + 2(40 - 2x)x = 60x + (80 - 4x)x = 60x + 80x - 4x^2 = -4x^2 + 140x$. You are trying to find $\displaystyle x$ such as $\displaystyle f(x) = 300$. Solve $\displaystyle -4x^2 + 140x = 300$ for $\displaystyle x$ :
$\displaystyle -4x^2 + 140x - 300 = 0$
Using the quadratic formula :
$\displaystyle \Delta = 140^2 - 4 \times (-4) \times (-300) = 14800$
$\displaystyle x_1 = \frac{-140 - \sqrt{14800}}{-8}$
$\displaystyle x_2 = \frac{-140 + \sqrt{14800}}{-8}$
A width can only be positive, so discard the negative solution. You just found the width of the concrete path
Does that make sense ?