A wedding hall has 8 cylindrical pillars, each 4 m tall and 70 cm across. Only the curved side of each pillar is to be painted, at ₹25 per square metre. What is the total cost? (Use π = 22/7.)

A pillar 70 cm across has a radius of 0.35 m, so its curved surface = 2πrh = $2\times (22/7)\times 0.35\times 4=8.8$ sq m. Eight pillars give 70.4 sq m, and at ₹25 that is ₹1760. The trap ₹3520 reads 70 cm as the RADIUS, doubling every curved surface; ₹220 prices a single pillar and forgets there are eight; ₹1914 also pays for the two circular ends of each pillar, 0.77 sq m per pillar, though those ends are buried in the floor and the ceiling and are never painted.