Selina Concise Class 8 Math Chapter 21 Surface Area Volume and Capacity Exercise 21B Solution
Concise Mathematics – Class 8 Selina Exercise 21B Solution by experts | 21B Selina Concise Solution Class 8 | Class 8 Selina Concise Ex 21B Page 240 – 241 Solution | RK Bansal Class 8 ICSE Book Solution Online.
(1) How many persons can be accommodated in a big-hall of dimensions 40 m, 25 m and 15 m assuming that each person requires 5 meter cube of air?
(2) The dimensions of a class-room are, length = 15 m, breadth = 12 m and a height = 7.5 m. Find, how many children can be accommodated in this class room, assuming 3.5 meter cube of air is needed for each child.
Solution: 
(3) The length, breadth and height of a room are 6 m, 5.4 m and 4 m respectively. Find the area of (i) its four walls (ii) its roof.
(4) A room 5 m long , 4.5 m wide and 3.6 m high has one door 1.5 m by 2.4 m and two windows each 1 m by 0.75 m. Find:
(i) the area of its walls, excluding doors and windows.
(ii) the cost of distempering its walls at the rate of Rs. 4.50 per meter square.
(iii) the cost of painting its roof at the rate of Rs. 9 per meter square.
(5) The dining hall of a hotel is 75 m long, 60 m broad and 16 m high. It has five doors 4 m by 3 m each and four windows 3 m by 1.6 m each. Find the cost of:
(i) papering its walls at the rate of Rs. 12 per meter square.
(ii) carpeting its floor at the rate of Rs. 25 per meter square.
(6) Find the volume of wood required to make a closed box of external dimensions 80 cm, 75 cm and 60 cm, the thickness of walls of the box being 2 cm throughout.
Solution: 
(7) A closed box measures 66 cm, 36 cm and 21 cm from outside. If its walls are made of metal sheet, 0.5 cm thick; find:
(i) the capacity of the box
(ii) volume of metal-sheet and
(iii) weight of the box, if 1 cm cube of metal weighs 3.6 g.
(8) The internal length, breadth and height of a closed box are 1 m, 80 cm and 25 cm respectively. If its sides are made of 2.5 cm thick wood; find:
(i) the capacity of the box
(ii) the volume of wood used to make the box.
(9) Find the area of metal-sheet required to make an open tank of length = 10 m, breadth = 7.5 m and depth = 3.8 m
Solution: 
(10) A tank 30 m long, 24 m wide and 4.5 m deep is to be made. It is open from the top. Find the cost of iron-sheet required, at the rate of Rs. 65 per meter square. to make the tank.
Solution: 
