A field in the shape of a circle has a boundary 220 $m$ long. Find the cost of ploughing it at ₹4 per $m^2$. Take π = 22/7.

$2\times (22/7)\times r=220$ gives $r=35m$, so the area = $(22/7)\times 35^2=3850m^2$ and the cost = $3850\times 4$ = ₹15400. (Charging the 220 $m$ boundary at ₹4 gives ₹880, but the rate is per square metre; solving π$r=220$ instead of 2π$r=220$ gives $r=70m$ and a wrong ₹61600.)