## Book: RD Sharma - Mathematics

### Chapter: 21. Mensuration-II (Volumes and Surface Areas of a Cuboid and a Cube)

#### Subject: Maths - Class 8th

##### Q. No. 17 of Exercise 21.3

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17
##### The central hall of a school is 80 m long and 8 m high. It has 10 doors each of size 3 m ×1.5 m and 10 windows each of size 1.5 m× 1 m. If the cost od white washing the walls of the hall at the rate of Rs. 1.20 per m2 is Rs. 2385.60, find the breadth of the hall.

Given,

Dimensions of central hall of a school = Length = 80 m , height = 8m

Let breadth of hall = b metre

So,

Area of each door = 3m × 1.5 m = 4.5 m2

Area of 10 doors = 10 × 4.5 = 45 m2

Area of each window = 1.5m × 1m = 1.5 m2

Area of 10 windows = 10 ×1.5 = 15 m2

Area occupied by doors and windows = 45 + 15 = 60 m2

Area of the walls of the hall including doors and windows =

= 2(80×8+b×8) = 2(640+8b) m2

Then,

Area of only walls = (area of walls including doors & windows – area occupied by doors & windows)

=

Total cost of white washing = Rs.2385.60

Given, Rate of white washing = Rs.1.20 per m2

So,

=

=

=

=

=

Hence,

Breadth of hall = 48 m

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