Perimeter of a rectangular shape is 20cm and it's diagonal is 8cm. Find the area.

The perimeter is given by $2(l+w) = 20$ cm.
This simplifies to $l+w = 10$ cm.

$l^2 + w^2 = d^2$.
We are given that the diagonal $d = 8$ cm, so $l^2 + w^2 = 8^2 = 64$ cm$^2$.

We know that $(l+w)^2 = l^2 + 2lw + w^2$.
We can substitute the values we have: $(10)^2 = (l^2 + w^2) + 2lw$.
$100 = 64 + 2lw$.

$100 - 64 = 2lw$
$36 = 2lw$
$lw = \frac{36}{2}$
$lw = 18$ cm$^2$.

The area of the rectangle is 18 cm$^2$.

18 cm^2