Q4 of 79 Page 229

A mansion has 12 right cylindrical pillars each having radius 50 cm and height 3.5 m. Find the cost to paint the lateral surface of the pillars at Rs 20 per square metre.

Given : Radius of cylindrical pillar = r = 50 cm = 0.5 m

Height of cylindrical pillar = h = 3.5 m


Number of cylindrical pillars = 12


Cost of painting per square metre = Rs 20


To find : The cost of painting the lateral surface of the pillars


Formula : CSA of cylinder = 2πrh


Solution : The lateral surface of a cylinder is the curved surface of the cylinder.


Area to be painted for one pillar = CSA of cylinder = 2πrh


= 2 × × 0.5 × 3.5


= 11 m2


Total area to be painted for 12 cylinders = 11 × 12 = 132 m2


Total cost of painting = Area to be painted × cost of painting per square meter


= 132 × 20 = Rs 2640


∴ The cost of painting 12 cylindrical pillars is Rs 2640.


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