NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.5 – Part 3
Q8 A solid cube of side is cut into eight cubes of equal volume. What will be the side of the new cube? Also, find the ratio between their surface areas.
Solution:
Edge of the cube
Therefore, volume of the cube
The big cube is cut into 8 equal smaller cubes, therefore volume of each of the smaller cube
Let, side of the smaller cube , then, we have or
Surface area of the cube
Therefore, ratio of their surface area = = or
Q9 A river deep and wide is flowing at the rate of per hour. How much water will fall into the sea in a minute?
Solution:

River depth

River width
Rate of flow of water is per hour. We need to convert this to m per minute according to the units used in our question. ( ) = per minute.
Therefore in one minute length of river falls in the sea. That is, in one minute a cuboid of length , width 40 m (the breadth of river) and height 3 m (depth of river) falls in the sea.
Volume of water flowing into the sea in a minute =