$A$ storage tank is in the form of a cube. When it is full of water,the volume of water is $15.625 \, m^3$. If the present depth of water is $1.3 \, m$,find the volume of water already used from the tank (in $m^3$).

Write True or False and justify your answer in each of the following: If a sphere is inscribed in a cube,then the ratio of the volume of the cube to the volume of the sphere will be $6: \pi$.

$A$ cylindrical tube open at both ends is made of an iron sheet which is $2 \, cm$ thick. If the outer diameter is $16 \, cm$ and its length is $100 \, cm$, find how many cubic centimeters of iron have been used in making the tube? (in $cm^3$)

State whether the following statement is true or false:
The ratio of the surface areas of two spheres is $81: 49$. Then,the ratio of their volumes is $729: 343$.

Two solid spheres made of the same metal have weights $5920\, g$ and $740\, g$ respectively. Determine the radius of the larger sphere,if the diameter of the smaller one is $5\, cm$. (in $cm$)

$A$ right triangle with sides $6 \, cm$,$8 \, cm$,and $10 \, cm$ is revolved about the side $8 \, cm$. Find the curved surface area of the solid so formed (in $cm^2$). (in $, cm^2$)