Quadratic Functions, Equations and Inequalities

A rectangular enclosure is built against a straight wall. No fencing is needed along the wall. Exactly \(40\) m of fencing is used for the other three sides.
Let each side perpendicular to the wall have length \(x\) m, and let the side parallel to the wall have length \(y\) m.

1. Express \(y\) in terms of \(x\), and state the possible range of \(x\).

2. Show that the area is \(A=40x-2x^2\).

3. Find the maximum area and the dimensions that achieve it.