Transcript text: 20. A rectangular enclosure is built using a long existing fence and three new sides that use a total of 40 metres of fencing.
existing long fence
Denote the length of the two new sides perpendicular to the existing fence by $x \mathrm{~m}$, and the length of the new side parallel to the existing fence by $y \mathrm{~m}$, so that $2 x+y=40$. Find the largest possible area of the enclosure.
$200 \mathrm{~m}^{2}$
$150 \mathrm{~m}^{2}$
$220 \mathrm{~m}^{2}$
$160 \mathrm{~m}^{2}$
$192 \mathrm{~m}^{2}$