Answer :

Let ABCD be a rhombus.

Area of the rhombus ABCD
= 2 × area of $\mathrm{?}ABD$ ... (i)

As we know that, in a rhombus, diagonals divides it in two equal parts.

In $\mathrm{?}ABD$, we have,
AB = 30 m, BD = 48 m, DA=30 m

Now, we know that,

$s=\frac{AB+BD+AD}{2}$
$?$ $s=\frac{30+48+30}{2}=\frac{108}{2}=54m$

Now, Area of triangle
= $\sqrt{54(54?30)(54?48)(54?30)}$
(Since, Heron's formula [area = $\sqrt{s(s?a)(s?b)(s?c)}$])

= $\sqrt{54\times 24\times 6\times 24}$
= $\sqrt{9\times 6\times 4\times 6\times 6\times 4\times 6}$
= $3\times 6\times 6\times 4m^2$
= $432m^2$

Thus, from Equation (i),

Area of rhombus
= 2 × 432m
= 864 m

Also, Number of cows = 18

So, therefore, Area of grass field per cow
= $\frac{864}{18}$ = $48m^2$.