A lawn is in the form of a rectangle whose sides are in the ratio 5 : 3 and its area is 3375 m2.

Let the length and breadth of the lawn be $5 x \mathrm{~m}$ and $3 x \mathrm{~m}$, respectively.

Now,

Area of the lawn $=5 x \times 3 x=5 x^{2}$

$\Rightarrow 15 x^{2}=3375$

$\Rightarrow x=\sqrt{\frac{3375}{15}}$

$\Rightarrow x=\sqrt{225}=15$

Length $=5 x=5 \times 15=75 \mathrm{~m}$

Breadth $=3 x=3 \times 15=45 \mathrm{~m}$

$\therefore$ Perimeter of the lawn $=2($ Length $+$ Breadth $)=2(75+45)=2 \times 120=240 \mathrm{~m}$

Total cost of fencing the lawn at Rs 20 per metre $=240 \times 20=$ Rs 4800