Imagine you had $40$ one-metre sections of fencing.

What is the largest rectangular area of land you could fence off?

Now imagine you could build your fence up against a wall, so you only need to use the fence for three sides of the enclosure:

What is the largest area you can fence off now?

Now imagine you can attach the fence to the wall shown below, at the point marked X.

What is the largest area you can fence off now?

Extension
Could you enclose even greater areas if you had $40$m of flexible wire fencing that could fence off curves as well as straight lines?