Find $(i)$ the curved surface area and $(ii)$ the total surface area of a hemisphere of radius $21\, cm$.

The slant height and base diameter of a conical tomb are $25 \, m$ and $14 \, m$ respectively. Find the cost of white-washing its curved surface at the rate of $Rs. 210$ per $100 \, m^2$.

$A$ hemispherical bowl is made of steel,$0.25 \, cm$ thick. The inner radius of the bowl is $5 \, cm$. Find the outer curved surface area of the bowl. (in $, cm^2$)

Find the curved surface area of a right circular cone whose slant height is $10\, cm$ and base radius is $7\, cm$. (in $, cm^{2}$)

Hameed has built a cubical water tank with a lid for his house,with each outer edge $1.5\, m$ long. He gets the outer surface of the tank,excluding the base,covered with square tiles of side $25\, cm$ (see figure). Find how much he would spend on the tiles,if the cost of the tiles is $Rs. 360$ per dozen.

$A$ corn cob (see figure),shaped somewhat like a cone,has the radius of its broadest end as $2.1\, cm$ and length (height) as $20\, cm$. If each $1\, cm^2$ of the surface of the cob carries an average of four grains,find how many grains you would find on the entire cob.