Volume of the rectangular water tank = 24 m3 l × b × h = 24 4 × 3 × h = 24 b = 2 m Inner surface area = 2 [(l × b)+ (b × b)] + (l × b) = 2(8 + 6)+12 = 28 + 12 = 40 m2 Outer surface area = 40 m2 [ignoring the thickness] Total cost of painting = 10 (40 + 40) = Rs 800