## Chapter 13 Surface Areas and Volumes Ex 13.9

**Question 1.** **A wooden bookshelf has external dimensions as follows : Height = 110cm, Depth = 25cm, Breadth = 85cm (see figure). The thickness of the plank is 5 cm everywhere. The external faces are to be polished and the inner faces are to be painted. If the rate of polishing-is 20 paise per cm ^{2} and the rate of pointing is 10 paise per cm^{2}, find the total expenses required for palishing and painting the surface of the bookshelf.**

**Solution:**

**Question 2.** **The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed on small supports as shown in 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}.**

**Solution:**

It is obvious, we have to subtract the cost of the sphere that is resting on the supports while calculating the cost of silver paint.

Surface area to be silver paint

**Question 3.**
**The diameter of a sphere is decreased by 25%. By what per cent does its curved surface area decrease?**
**Solution:**

Let d be the diameter of the sphere