$A$ village,having a population of $4000$,requires $150$ litres of water per head per day. It has a tank measuring $20 \,m \times 15 \,m \times 6 \,m$. For how many days will the water of this tank last?

$1\, m^{3} = \dots \, cm^{3}$

State whether the following statement is true or false:
The radius of a cone can be $3 \, cm$,slant height $4 \, cm$,and height $5 \, cm$.

The ratio of the curved surface area and the total surface area of a cylinder is $2: 5$. Find the ratio of the radius and height of the cylinder.

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.

$A$ wall of length $10\, m$ was to be built across an open ground. The height of the wall is $4\, m$ and thickness of the wall is $24\, cm$. If this wall is to be built up with bricks whose dimensions are $24\, cm \times 12\, cm \times 8\, cm$,how many bricks would be required?