RD Sharma Class 8 Solutions Chapter 21 Mensuration Ex 21.2



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TextbookNCERT
ClassClass 8
SubjectMaths
ChapterChapter 21
Chapter NameMensuration
ExerciseEx 21.2

RD Sharma Solutions for Class 8 Chapter 21 Mensuration Ex 21.2 Download PDF

Question 1.
Find the volume in cubic metres (cu.m) of each of the cuboids whose dimensions are :
(i) length = 12 cm, breadth = 10 m, height = 4.5 m
(ii) length = 4 m, breadth = 2.5 m, height = 50 cm
(iii) length = 10 m, breadth = 25 dm, height = 25 cm.
Solution:
(i) Length of cuboid (l) = 12 m
Breadth (b) = 10m
and height (h) = 4.5 m
∴Volume = l x b x h = 12 x 10 x 4.5 m3
= 540 m3
(ii) Length of cuboid (l) = 4 m
Breadth (b) = 2.5m
Height (h) = 50 cm = 0.5 m
∴ Volume = l x b x h = 4 x 2.5 x 0.5 = 5 m3
(iii) Length of cuboid (l) = 10 m
Breadth (b) = 25 dm = 2.5 m
Height (h) = 25 cm 0.25 m
∴ Volume = l x b x h = 10 x 2.5 x 0.25 m3 = 6.25 m3

Question 2.
Find the volume in cubic decimetre of each of the cubes whose side is
(i) 1.5 m
(it) 75 cm
(iii) 2 dm 5 cm
Solution:
(i) Side of cube (a) = 1.5 m
∴ Volume = a3 = (1.5)3 m3
= 1.5 x 1.5 x 1.5 m3 = 3.375 m3
= 3.375 x 1000 = 3375 dm3
(ii) Side of cube (a) = 75 cm = 7.5 dm
∴ Volume = a3 = (7.5)3 dm3
= 421.875 dm3
(iii) Side of cube (a) = 2 dm 5 cm = 2.5 dm
∴ Volume = (a)3 = (2.5)3 dm3
= 15.625 dm3

Question 3.
How much clay is dug out in digging a well measuring 3m by 2m by 5m?
Solution:
Length of well (l) = 3m
breadth (b) = 2 m
and height (depth) (h) = 5 m
Volume of earth dug out = l x b x h = 3 x 2 x 5 = 30m3

Question 4.
What will be the height of a cuboid of volume 168 m3, if the area of its base is 28 m2 ?
Solution:
Volume of a cuboid = 168 m3
Area of its base l.e., l x b = 28 m3
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 1

Question 5.
A tank is 8 m long, 6 m broad and 2 m high. How much water can it contain ?
Solution:
Length of tank (l) = 8 m
Breadth (b) = 6 m
Height (h) = 2 m
∴ Volume of water in the tank = l x b x h = 8 x 6 x 2 = 96 m3
= 96 x 1000 = 96000litres (∵1m3 = 1000litre)

Question 6.
The capacity of a certain cuboidal tank is 50000 litres of water. Find the breadth of the tank if its height and length are 10 m and 2.5 m respectively.
Solution:
Capacity of water in the tank = 50000 litres
∴ Volume of water = 50000 x 11000 = 50 m3 (1000 l = 1 m3)
Height of tank (h)= 10 m
and length (l) = 2.5 m
Volume 50
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 2

Question 7.
A rectangular diesel tanker is 2 m long, 2 m wide and 40 cm deep. How many litres of diesel can it hold ?
Solution:
Length of tanker (l) = 2 m
Breadth (b) = 2m
Depth (h) = 40 cm = 0.4 m
∴ Volume = l x bx h = 2 x 2 x 0.4=1.6m3
Quantity of diesel = 1.6 x 1000 litres (1 m3= 1000 l)
= 1600 litres

Question 8.
The length, breadth and height of a room are 5 m, 4.5 m and 3 m, respectively. Find the volume of the air it contains.
Solution:
Length of room (l) = 5 m
Breadth (6) = 4.5 m
and height (h) = 3 m
∴ Volume of air it contains
= l x b x h = 5 x 4.5 x 3 m3
= 67.5 m3

Question 9.
A water tank is 3 m long, 2 m broad and 1 m deep. How many litres of water can it hold ?
Solution:
Length of tank (l) = 3 m
Breadth (b) = 2 m
and depth (h) = 1 m
∴ Volume of tank = l x b x h
= 3 x 2 x 1 = 6 m3
∴ Quantity of water it can contains
= 6 x 1000 litres = 6000 litres (1 m3= 1000 litres)

Question 10.
How many planks each of which is 3 m long, 15 cm broad and 5 cm thick can be prepared from a wooden block 6 m long, 75 cm broad and 45 cm thick ?
Solution:
Length of wooden block (l) = 6 m
Width (b) = 75 cm = 0.75 m
Thickness (h) = 45 cm = 0.45 m
∴ Volume = l x b x h = 6 x 0.75 x 0.45 m3
Length of plank (l) = 3 m
Breadth (b) = 15 cm = 0.15 m
Thickness (h) = 5 cm = 0.05 m
∴ Volume = 3 x 0.15 x 0.05 m3
Number of planks
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 3

Question 11.
How many bricks each of size 25 cm x 10 cm x 8 cm will be required to build a wall 5 m long, 3 m high and 16 cm thick assuming that the volume of sand and cement used in the construction is negligible ?
Solution:
Size of one brick = 25 cm x 10 cm x 8 cm
∴ Volume of one brick = 25 x 10 x 8 cm3
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 4
Length of wall (l) = 5 m
Width (b) = 0.16 m
Height (h) = 3 m
∴ Volume of wall = l x b x h
= 5 x 0.16 x 3 m3 = 2.4 m3
∴ Number of bricks required
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 5

Question 12.
A village, having a population of 4000 requires 150 litres water per head per day. It has a tank which is 20 m long, 15 m broad and 6 m high. For how many days the water of this tank will last ?
Solution:
Total population of a village = 4000
Water required for each person for one day = 150 litres
∴ Water required for 4000 persons for one day = 150 x 4000 = 600000 litres
Length of tank (l) = 20 m
Breadth (b) = 15 m
Height (h) = 6 m
∴ Volume of tank = l x b x h = 20 x 15 x 6 m3 = 1800 m3
Capacity of water in the tank = 1800 x 1000 l= 1800000l (1 m3 = 1000 l)
∴ Number of days, the water will last
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 6

Question 13.
A rectangular field is 70 m long and 60 m broad. A well of dimensions 14 m x 8 m x 6 m is dug outside the field and the earth dugout from this well is spread evenly on the field. How much will the earth level rise ?
Solution:
Length of well (l) = 14 m
Breadth (A) = 8m
Depth (A) = 6m
∴ Volume of earth dugout = l x bx h
= 14 x 8 x 6 = 672 m3 Length of field = 70 m
and breadth = 60 m
Let h be the height of earth spread over
Then 70 x 60 x h = 672
⇒ h = 67270×60 = 0.16m
∴ Height of earth = 0.16 m = 16 cm

Question 14.
A swimming pool is 250 m long and 130 m wide. 3250 cubic metres of water is pumped into it. Find the rise in the level of water.
Solution:
Volume of water = 3250 m3
Length of pool (l) = 250 m
Breadth (b)= 130 m
∴ Height of water level
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 7

Question 15.
A beam 5 m long and 40 cm wide contains 0.6 cubic metres of wood. How thick is the beam?
Solution:
Volume of wood of the beam = 0.6 m3 = 600000
Length of beam (l) = 5 m = 500 cm
Breadth (b) = 40 cm
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 8

Question 16.
The rainfall on a certain day was 6 cm. How many litres of water fell on 3 hectares of field on that day ?
Solution:
Area of the field = 3 hectares
= 3 x 10000 square metres
= 30000 square metres
Height of rainfall = 6 cm = m3
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 9

Question 17.
An 8 m long cuboidal beam of wood when sliced produces four thousand 1 cm cubes and there is no wastage of wood in this process. If one edge of the beam is 0.5 m, find the third edge.
Solution:
Length of cuboidal beam (l) = 8 m = 800 cm
Number of cubical sliced = 4000
Edge of each cube = 1 cm
Volume of beam = 4000 (1)3 cm3 = 4000 cm3
One edge of the beam = 0.5 m = 50 cm.
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 10

Question 18.
The dimensions of a metal block are 2.25 m by 1.5 m by 27 cm. It is melted and recast into cubes, each of the side 45 cm. How many cubes are formed ?
Solution:
Dimensions of metal block = 2.25 m x 1.5 m x 27 cm
∴ Volume = 2.25 x 1.5 x 0.27 m3
= 225 x 150 x 27 cm3 = 911250 cm3
Side of each cube (a) = 45 cm
∴ Volume of one cube = a3 = (45)3 cm3 = 91125 cm3
∴ Number of cubes = 91125091125 = 10

Question 19.
A solid rectangular piece of iron measures 6 m by 6 cm by 2 cm. Find the weight of this piece if 1 cm3 of iron weighs 8 gm.
Solution:
Dimensions of a piece of rectangular iron = 6m x 6cm x 2cm
∴ Volume = 600 x 6 x 2 cm3 = 7200 cm3
Weight of 1 cm3 = 8 gm
∴ Total weight of the piece = 7200 x 8 gm
= 57600 gm = 576001000 kg = 57.6 kg

Question 20.
Fill in the blanks in each of the following so as to make the statement true :
(i) 1 m3 = ……… cm3
(ii) 1 litre = …….. cubic decimetre
(iii) 1 kl = …… m3
(iv) The volume of a cube of side 8 cm is …….. .
(v) The volume of wooden cuboid of length 10 cm and breadth 8 cm is 4000 cm3. The height of the cuboid is …….. cm
(vi) 1 cu.dm = …….. cu.mm
(vii) 1 cu.km = ……cu.m
(viii) 1 litre =…….. cu.cm
(ix) 1 ml = ……… cu.cm
(x) 1 kl = ……… cu.dm = ……. cu.cm.
Solution:
(i) 1 m3 = 1000000 or 10cm3
(ii) 1 litre = 1 cubic decimetre
(iii) 1 kl = 1 m3
(iv) The volume of a cube of side 8 cm is 512 cm3 (V = a3 = 8 x 8 x 8 = 512 cm3)
(v) The volume of a wooden cuboid of length 10 cm and breadth 8 cm is 4000 cm3. The height of the cuboid is 50 cm
RD Sharma Class 8 Solutions Chapter 21 Mensuration II Ex 21.2 11
(vi) 1 cu.dm = 1000000 cu mm = 106 cu.mm
(vii) 1 cu.km = 1000 x 1000 x 1000 cu.m = 109 cu.m
(viii) 1 litre = 1000 cu.cm = 103 cu.cm
(ix) 1 ml = 1 cu.cm
(x) 1 kl = 1000 cu.dm = 100 x 100 x 100 cu.cm = 106 cu.cm

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