A rectangular field has area $3840{\text{m}}^2$ and its length and breadth are in the ratio 5 : 3. Find the cost of fencing it right round at ₹18 per metre.

Let the sides be $5x$ and $3x$, so $15x^2=3840,x^2=256$ and $x=16$. The field is 80 m by 48 m and its perimeter = $2(80+48)=256$ m. Cost = $256\times 18$ = ₹4608. (Charging the area at the per-metre rate, $3840\times 18$, wrongly gives ₹69120; using only half the perimeter gives ₹2304.)