Ncert mathematics solutions class 9 Solutions for Chapter 13 surface areas and volumes

A plastic box 1.5 m long, 1.25 m wide, and 65 cm deep, is to be made. It is to be open at the top.

Ignoring the thickness of the plastic sheet, determine:

(i) The area of the sheet required for making the box.

(ii) The cost of the sheet for it, if a sheet measuring 1 m^2 costs Rs. 20.

The length, breadth and height of a room are 5 m, 4 m and 3 m respectively. Find the cost of

white washing the walls of the room and ceiling at the rate of Rs 7.50 per \mathrm{m}^{2}

The floor of a rectangular hall has a perimeter 250 m. If the cost of painting the four walls at

\text { the rate of } R \text { s. } 10 \text { per } m^{2} \text { is } R \text { s. } 15000, \text { find the height of the hall. }

The paint in a certain container is sufficient to paint an area equal to 9.375 \mathrm{m}^{2}. How many

bricks of dimensions 22.5 cm × 10 cm × 7.5 cm can be painted out of this container?

A cubical box has each edge 10 cm and another cuboidal box is 12.5 cm long, 10 cm wide and 8

cm high

(i) Which box has the greater lateral surface area and by how much?

(ii) Which box has the smaller total surface area and by how much?

A small indoor greenhouse (herbarium) is made entirely of glass panes (including base) held

together with tape. It is 30 cm long, 25 cm wide and 25 cm high.

(i) What is the area of the glass?

(ii) How much of tape is needed for all the 12 edges?

Shanti Sweets Stall was placing an order for making cardboard boxes for packing their sweets.

Two sizes of boxes were required. The bigger of dimensions 25 cm × 20cm × 5cm and the smaller

of dimension 15cm × 12cm × 5cm. For all the overlaps, 5% of the total surface area is required

extra. If the cost of the cardboard is Rs. 4 for 1000 \mathrm{cm}^{2}, find the cost of cardboard required for

supplying 250 boxes of each kind.

Praveen wanted to make a temporary shelter for her car, by making a box-like structure with

the tarpaulin that covers all the four sides and the top of the car ( with the front face as a flap which

can be rolled up). Assuming that the stitching margins are very small, and therefore negligible,

how many tarpaulins would be required to make the shelter of height 2.5m, with base dimensions

4m × 3m?

The curved surface area of a right circular cylinder of height 14 cm is 88 \mathrm{cm}^{2}. Find the diameter

of the base of the cylinder. (Assume π = 22/7 )

It is required to make a closed cylindrical tank of height 1m and base diameter 140 cm from a

metal sheet. How many square meters of the sheet are required for the same? Assume π=22/7

A metal pipe is 77 cm long. The inner diameter of a cross-section is 4 cm, the outer diameter

being 4.4cm. (see fig. 13.11). Find its

(i) inner curved surface area,

(ii) outer curved surface area

(iii) total surface area

(Assume π=22/7)

The diameter of a roller is 84 cm and its length is 120 cm. It takes 500 complete revolutions to

move once over to level a playground. Find the area of the playground in \mathrm{m}^{2} ? (Assume π=22/7)

Curved surface area of a right circular cylinder is 4.4 \mathbf{m}^{2}. If the radius of the base of

the cylinder is 0.7 m, find its height. (Assume π=22/7)

The inner diameter of a circular well is 3.5 m. It is 10 m deep. Find

(i) its inner curved surface area,

(ii) the cost of plastering this curved surface at the rate of Rs. 40 per \mathbf{m}^{2}

(Assume π=22/7)

In a hot water heating system, there is a cylindrical pipe of length 28 m and diameter of 5 cm. Find

the total radiating surface in the system. (Assume π=22/7)

Find

(i) the lateral or curved surface area of a closed cylindrical petrol storage tank that is 4.2 m in

diameter and 4.5m high.

(ii) How much steel was actually used, if 1/12 of the steel actually used was wasted in making the

tank. (Assume π=22/7)

In fig. 13.12, you see the frame of a lampshade. It is to be covered with a decorative cloth.

The frame has a base diameter of 20 cm and a height of 30 cm. A margin of 2.5 cm is to be given for

folding it over the top and bottom of the frame. Find how much cloth is required for covering the

lampshade. (Assume π=22/7)

The students of Vidyalaya were asked to participate in a competition for making and

decorating penholders in the shape of a cylinder with a base, using cardboard. Each penholder

was to be of radius 3 cm and height 10.5 cm. The Vidyalaya was to supply the competitors with

cardboard. If there were 35 competitors, how much cardboard was required to be bought for the

competition? (Assume π=22/7)

Diameter of the base of a cone is 10.5 cm and its slant height is 10 cm. Find its curved

surface area (Assume π = 22/7)

Find the total surface area of a cone, if its slant height is 21 m and diameter of its base is 24

m. (Assume π = 22/7)

The curved surface area of a cone is 308 \mathrm{cm}^{2} and its slant height is 14 cm. Find

(i) radius of the base and (ii) total surface area of the cone.

(Assume π=22/7)

A conical tent is 10 m high and the radius of its base is 24 m. Find

(i) slant height of the tent.

(ii)cost of the canvas required to make the tent, if the cost of 1 m2 canvas is Rs 70.

(Assume π=22/7)

What length of tarpaulin 3 m wide will be required to make conical tent of height 8 m and

base radius 6m? Assume that the extra length of material that will be required for stitching

margins and wastage in cutting is approximately 20 cm. [Use π=3.14]

The slant height and base diameter of the conical tomb is 25 m and 14 m respectively. Find the

cost of white-washing its curved surface at the rate of Rs. 210 per 100 \mathrm{m}^{2} . \text { (Assume } \left.\pi=22 / 7\right)

A joker’s cap is in the form of right circular cone of base radius 7 cm and height 24 cm.

Find the area of the sheet required to make 10 such caps. (Assume π=22/7)

A bus stop is barricaded from the remaining part of the road, by using 50 hollow cones

made of recycled cardboard. Each cone has a base diameter of 40 cm and a height of 1 m. If the

outer side of each of the cones is to be painted and the cost of painting is Rs. 12 per \mathbf{m}^{2}

what will be the cost of painting all these cones? (Use π = 3.14 and take √(1.04) = 1.02)

Find the surface area of a sphere of radius:

(i) 10.5cm (ii) 5.6 cm (iii) 14cm

(Assume π=22/7)

Find the surface area of a sphere of diameter:

(i) 14 cm (ii) 21 cm (iii) 3.5 cm

(Assume π=22/7)

Find the total surface area of a hemisphere of radius 10 cm. [Use π=3.14]

Find the cost of digging a cuboidal pit 8 m long, 6 m broad, and 3 m deep at the rate of Rs 30 per \mathbf{m}^{3}

A hemispherical bowl made of brass has an inner diameter of 10.5 cm. Find the cost of tin-plating it

on the inside at the rate of Rs 16 per 100 \mathrm{cm}^{2} . \text { (Assume } \left.\pi=22 / 7\right)

Find the radius of a sphere whose surface area is 154 \mathrm{cm}^{2} . \text { (Assume } \left.\pi=22 / 7\right)Let the radius of the sphere be r.

The diameter of the moon is approximately one-fourth of the diameter of the earth.

Find the ratio of their surface areas.

A hemispherical bowl is made of steel, 0.25 cm thick. The inner radius of the bowl is 5cm. Find

the outer curved surface of the bowl. (Assume π = 22/7)

A right circular cylinder just encloses a sphere of radius r (see fig. 13.22). Find

(i) the surface area of the sphere,

(ii) the curved surface area of the cylinder,

(iii) the ratio of the areas obtained in (i) and (ii).

A river 3 m deep and 40 m wide is flowing at the rate of 2 km per hour. How much water will

fall into the sea in a minute?

A matchbox measures 4 cm x 2.5 cm x 1.5 cm.What will be the volume of a packet

containing 12 such boxes?

A cuboidal water tank is 6 m long, 5 m wide and 4.5 m deep. How many liters of water can

Does it hold? \left(1 \mathrm{m}^{3}=1000 \mathrm{l}\right)

A cuboidal vessel is 10 m long and 8 m wide. How high must it be made to hold 380 cubic meters

of a liquid?

The capacity of a cuboidal tank is 50000 litres of water. Find the breadth of the tank, if its

length and depth are respectively 2.5 m and 10 m.

A village, having a population of 4000, requires 150 litres of water per head per day.

It has a tank measuring 20 m x 15 m x 6 m. For how many days will the water of this tank last?

A godown measures 40 m x 25 m x 15 m. Find the maximum number of wooden crates each

measuring 1.5 m x 1.25 m x 0.5 m that can be stored in the godown.

A solid cube of side 12 cm 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.

The circumference of the base of the cylindrical vessel is 132 cm and its height is 25 cm.

How many liters of water can it hold? \left(1000 \mathrm{cm}^{3}=1 \mathrm{L}\right) \text { (Assume } \left.\pi=22 / 7\right)

The inner diameter of a cylindrical wooden pipe is 24 cm and its outer diameter is 28 cm. The

length of the pipe is 35 cm. Find the mass of the pipe, if 1 \mathrm{cm}^{3} of wood has a mass of 0.6g. (Assume

π = 22/7)

A soft drink is available in two packs – (i) a tin can with a rectangular base of length 5 cm and

width 4 cm, having a height of 15 cm and (ii) a plastic cylinder with circular base of diameter 7 cm

and height 10 cm. Which container has greater capacity and by how much? (Assume π = 22/7)

If the lateral surface of a cylinder is 94.2 \mathrm{cm}^{2} and its height is 5 cm, then find

It costs Rs 2200 to paint the inner curved surface of a cylindrical vessel 10 m deep. If the cost of

painting is at the rate of Rs 20 per \mathbf{m}^{2}, \mathbf{1} find

(i) inner curved surface area of the vessel

(ii) radius of the base

(iii) capacity of the vessel

(Assume π=22/7)

The capacity of a closed cylindrical vessel of height 1 m is 15.4 liters. How many square meters

of metal sheet would be needed to make it? (Assume π = 22/7)

A lead pencil consists of a cylinder of wood with a solid cylinder of graphite filled in the

interior. The diameter of the pencil is 7 mm and the diameter of the graphite is 1 mm. If the

length of the pencil is 14 cm, find the volume of the wood, and that of the graphite. (Assume π

= 22/7)

A patient in a hospital is given soup daily in a cylindrical bowl of diameter 7cm. If the bowl is

filled with soup to a height of 4cm, how much soup the hospital has to prepare daily to serve 250

patients? (Assume π=22/7)

Find the volume of the right circular cone with

(i) radius 6 cm, height 7 cm (ii) radius 3.5 cm, height 12 cm (Assume π = 22/7)

Find the capacity in litres of a conical vessel with

(i) radius 7 cm, slant height 25 cm (ii) height 12 cm, slant height 12 cm

(Assume π=22/7)

The height of a cone is 15 cm. If its volume is 1570 \mathrm{cm}^{3} find the diameter of its base. (Use π

=3.14)

If the volume of a right circular cone of height 9 cm is 48 π \mathrm{cm}^{3}, , find the diameter of its base.

A conical pit of top diameter 3.5 m is 12 m deep. What is its capacity in kiloliters?

(Assume π = 22/7)

The volume of a right circular cone is 9856 \mathrm{cm}^{3}. .If the diameter of the base is 28 cm, find

(i) height of the cone

(ii) slant height of the cone

(iii) curved surface area of the cone

(Assume π=22/7)

A right triangle ABC with sides 5 cm, 12 cm and 13 cm is revolved about the side 12 cm. Find

the volume of the solid so obtained.

If the triangle ABC in the Question 7 is revolved about the side 5 cm, then find the volume of

the solids so obtained. Find also the ratio of the volumes of the two solids obtained in Questions 7

and 8.

A heap of wheat is in the form of a cone whose diameter is 10.5 m and height is 3 m. Find its

volume. The heap is to be covered by canvas to protect it from rain. Find the area of the canvas.

(Assume π = 22/7)

Find the volume of a sphere whose radius is

(i) 7 cm (ii) 0.63 m

(Assume π=22/7)

Find the amount of water displaced by a solid spherical ball of diameter

(i) 28 cm (ii) 0.21 m

(Assume π=22/7)

The diameter of a metallic ball is 4.2 cm. What is the mass of the ball, if the density of the metal

is 8.9 g per cm3? (Assume π = 22/7)

The diameter of the moon is approximately one-fourth of the diameter of the earth. What

fraction of the volume of the earth is the volume of the moon?

. How many litres of milk can a hemispherical bowl of diameter 10.5 cm hold? (Assume π = 22/7)

.A hemispherical tank is made up of an iron sheet 1 cm thick. If the inner radius is 1 m, then

find the volume of the iron used to make the tank. (Assume π = 22/7)

Find the volume of a sphere whose surface area is 154 \mathrm{cm}^{2} . \text { (Assume } \left.\pi=22 / 7\right)

A dome of a building is in the form of a hemi sphere. From inside, it was white-washed at the

cost of Rs. 4989.60. If the cost of white-washing is Rs 20 per square meter, find the

(i) inside surface area of the dome, (ii) volume of the air inside the dome

(Assume π = 22/7)

Twenty-seven solid iron spheres, each of radius r and surface area S are melted to form a sphere

with surface area S’. Find the

(i) radius r’ of the new sphere,

(ii) the ratio of S and S’.

A capsule of medicine is in the shape of a sphere of diameter 3.5 mm. How much medicine (in \mathbf{m} \mathbf{m}^{3} \mathbf{)}

is needed to fill this capsule? (Assume π = 22/7)

A wooden bookshelf has external dimensions as follows: Height = 110 cm, Depth = 25 cm,

Breadth = 85 cm (see fig. 13.31). The thickness of the plank is 5cm 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 cm2

and the rate of painting is 10 paise per cm2, find the total expenses required for polishing and

painting the surface of the bookshelf.

The front compound wall of a house is decorated by wooden spheres of diameter 21 cm, placed

on small supports as shown in fig. 13.32. Eight such spheres are used forth is 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 cm2 and black paint costs 5

paise per \mathrm{cm}^{2}

The diameter of a sphere is decreased by 25%. By what percent does its curved surface area

decrease?