A field is shaped like a trapezium whose parallel sides are 25 m and 15 m, the perpendicular distance between them being 12 m. What is the cost of levelling the field at ₹18 per square metre?

Area = (1/2) $\times$ (sum of the parallel sides) $\times$ (distance between them) = $(1/2)\times (25+15)\times 12=240$ sq m, so the cost = $240\times 18$ = ₹4320. The trap ₹8640 drops the 1/2; ₹1080 uses the DIFFERENCE 25 - 15 in place of the sum; ₹6750 multiplies the two parallel sides together, $25\times 15=375$ sq m, as though the field were a rectangle.