The radius of a cone is $10\, cm$ and the total surface area of a cone is $880\, cm^2$. Find the slant height in $cm$. (Use $\pi = 22/7$)

The length of a metal hollow cylinder is $70\, cm$. If the radius of the hollow inside it is $10\, cm$ and the metal thickness is $1\, cm$,find the difference between the areas of the outer and inner curved surfaces.

Curved surface area of a hemisphere $= \ldots \ldots \ldots$

Write True or False and justify your answer: The volume of a sphere is equal to two-thirds of the volume of a cylinder whose height and diameter are equal to the diameter of the sphere.

The front compound wall of a house is decorated by wooden spheres of diameter $21\, cm,$ placed on small supports as shown in the figure. Eight such spheres are used for this purpose,and are to be painted silver. Each support is a cylinder of radius $1.5\, cm$ and height $7\, cm$ and is to be painted black. Find the cost of paint required if silver paint costs $25$ paise per $cm^{2}$ and black paint costs $5$ paise per $cm^{2}.$

$A$ cylindrical roller $2.5\, m$ in length and $1.75\, m$ in radius,when rolled on a road,was found to cover an area of $5500\, m^{2}$. How many revolutions did it make?