The internal and external diameter of a hollow hemispherical vessel are 20 cm and 28 cm respectively. Find the cost of painting the vessel all over at 35 paise by `cm^(2)`

Explanation

Outer radius of the vessel, R = 14 cm
Inner radius of the vessel, r = 10 cm
Area of the outer surface `= (2pi R^(2))` sq units
`= (2pi xx 14 xx 14) cm^(2)= (392pi) cm^(2)`
Area of the inner surface `= (2pir^(2))` sq units
`= (2pi xx 10 xx 10) cm^(2) = (200pi) cm^(2)`
Area of the ring at the top `= pi (R^(2) - r^(2))` sq units
`= pi [(14)^(2) - (10)^(2)] cm^(2)`
`= pi (14 + 10) (14 - 10) cm^(2) = (96pi) cm^(2)`
Total area to be painted `= (392pi + 200pi + 96pi) cm^(2) = (688pi) cm^(2)`
Cost of painting = Rs `(688pi xx (35)/(100)) = Rs (688 xx (22)/(7) xx (35)/(100))`
`= Rs (3784)/(5) = Rs 756.80`