Class 9 Maths

Surface Area

Exercise 13.1

Part 1

Question 1: 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:

1. The area of the sheet required for making the box.
2. The cost of sheet for it, if a sheet measuring 1m2 costs Rs 20.

Answer: l=1.5 m, b=1.25 m, h=0.65 m

Area of base =l\xx\b=1.5xx1.25=1.875 sq m

Area of four walls =2h(l+b)=2xx0.65(1.5+1.25)

=1.3 × 2.75 = 3.575 sq m

Total area = 1.875 + 3.575 = 5.45 sq m

Cost = Rate × Area = 20 × 5.45 = Rs. 109

Question 2: 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 the ceiling at the rate of Rs 7.50 per m2

Answer: l = 5m , b = 4m, h = 3 m, rate = Rs. 7.50 per sq m

Area of roof =l\xx\b=5xx4=20 sq m

Area of four walls =2h(l+b)=2xx3(5+4)

=6xx9=54 sq m

Total area = 20 + 54 = 74 sq m

Cost = Rate × Area =7.50xx74 = Rs. 555

Question 3: The floor of a rectangular hall has a perimeter 250 m. If the cost of painting the four walls at the rate of Rs 10 per m2 is Rs 15000, find the height of the hall. [Hint : Area of the four walls = Lateral surface area.]

Answer: Perimeter = 250m, rate = Rs. 10 per sq m, cost = Rs. 15000, h = ?

Area = Cost ÷ Rate =(1500)/(10)=1500 sq m

Area of four walls = height × perimeter

Or, 1500=h\xx250

Or, h=(1500)/(250)=6 m

Question 4: The paint in a certain container is sufficient to paint an area equal to 9.375 m2. How many bricks of dimensions 22.5 cm × 10 cm × 7.5 cm can be painted out of this container?

Answer: l = 22.5 cm, b = 10 cm, h = 7.5 cm, total area = 9.375 sq m = 93750 sq cm

Surface area of brick =2(lb+bh+lh)

=2(22.5xx10+10xx7.5+22.5xx7.5)

=2(225+75+168.75)

=2xx468.75=937.50 sq cm

Hence, number of bricks = 93750/937.50 = 100