**Selina Concise Class 7 Math Chapter 20 Mensuration Exercise 20B Solution**

**EXERCISE 20B**

(1) Area of the rectangle = (25 × 16) cm^{2} = 400 cm^{2}

(2) Here, diagonal = 1 m = 100 cm

Let the breadth of rectangular broad be x cm.

Applying Pythagoras theorem,

∴ AC^{2} = AB^{2} + BC^{2}

⇒ (100)^{2} = (96)^{2} + x^{2}

⇒ x^{2} = (100)^{2} – (96)^{2}

⇒ x^{2} = 10000 – 9216

⇒ x^{2} = 784

⇒ x = √784 = 28

Breadth of the board = 28 cm

∴ Its area = (96 × 28) cm^{2} = 2688 cm^{2}

(3) Let the length and breadth will be 4x and 3x m

Its area = (4x × 3x) = 12x^{2} m^{2}

Given area = 1728 m^{2}

∴ 12x^{2} = 1728

⇒ x^{2} = 1728/12

⇒ x^{2} = 144

⇒ x = √144 = 12

Its perimeter = 2 × [(4 × 12) + (3 × 12)] m

= 2 × (48 + 36) = 2 × 84 = 168 m

(ii) Rate of fencing = Rs 40

∴ Total cost of fencing = Rs (168 × 40) = Rs 6720

(4) Area of the floor = (40 × 15) m^{2} = 600 m^{2}

Length of each tile = 60 cm = 0.6 m

And breadth = 50 cm = 0.5 m

∴ Area of each tile = (0.6 × 0.5) = 0.3 m^{2}

∴ Number of tiles = (600×10)/3=2000

(9) Area of the parallelogram = (20 × 16) = 320 cm^{2}

(10) Let the height of the parallelogram = x and its base = x/3

∴ x × x/3 = 768

⇒ x^{2} = 768 × 3

⇒ x^{2} = 2304

⇒ x = √2304 = 48

Hence, the base = 48 cm and height = 48/3 = 16 cm

(15) Area of the rhombus = 84 cm^{2}

Perimeter = 56 cm

∴ Its side = 56/4 = 14 cm

∴ Height = 84/14 = 6 cm

(16) Area of triangle = ½ (base × height)

= ½ × 30 × 18 = 270 cm^{2}

(17) Let the height of the triangle be x cm.

∴ ½ × 18 × x = 270

⇒ 18x = (270 × 2) = 540

⇒ x = 540/18

⇒ x = 30 cm

(18) Length of the other leg = (160 × 2)/16 = 20 cm

(19) Let the other leg will be x cm.

Applying Pythagoras theorem,

AC^{2} = AB^{2} + BC^{2}

⇒ (13)^{2} = (12)^{2} + x^{2}

⇒ x^{2} = (13)^{2} – (12)^{2}

⇒ x^{2} = 169 – 144

⇒ x = √25 = 5 cm

∴ Area = ½ × 12 × 5 = 6 × 5 = 30 cm^{2}

__Get next Exercise 21A solutions click here__

Answer of 36 is wrongly done

What a good math

Thank you so much for your explanation

You welcome

I appreciate all your answers but the answer number 37 is wrong.

Ok will check