Surface Area of a Cuboid and a Cube

Water flows in a tank $150 \mathrm{m}\times 100 \mathrm{m}$ at the base, through a pipe whose cross-section is $2 \mathrm{dm}$ by $1.5 \mathrm{dm}$ at the speed of $15 \mathrm{km}$ per hour. In what time, will the water be $3$ metres deep?

The edge of a cube is $20\mathrm{cm}$. How many small cubes of edge $5\mathrm{cm}$ can be formed from this cube?

A metal sheet $27\mathrm{cm}$ long. $8\mathrm{cm}$ broad and $1\mathrm{cm}$ thick is melted into a cube. Find the difference between surface areas of two solids.