Selina Concise Class 7 Math Chapter 12 Simple Linear Equations Exercise 12D Solution

Selina Concise Class 7 Math Chapter 12 Simple Linear Equations Exercise 12D Solution

EXERCISE 12D

(1) Let the number be x.

∴ x/5 = 5

⇒ x = 5 × 5

⇒ x = 25

Hence, the number is 25.

(2) Let the number be x.

∴ 6x = 72

⇒ x = 72/6

⇒ x = 12

Hence, the number is 12.

(3) Let the number be x.

∴ x + 15 = 69

⇒ x = 69 – 15

⇒ x = 54

Hence, the number is 54.

(4) Let the number be 2x.

∴ 2x + 4 = 80

⇒ 2x = 80 – 4

⇒ x = 76/2

⇒ x = 38

Hence, the number is 38.

(7) Let the number be x.

∴ x – 35 = 53 – x

⇒ x + x = 53 + 35

⇒ 2x = 88

⇒ x = 88/2

⇒ x = 44

(8) Let the numbers are x and 2x.

∴ x + 2x = 18

⇒ 3x = 18

⇒ x = 18/3

⇒ x = 6

Hence, the numbers are 6 and (2 × 6) = 12.

(9) Let the numbers are x and (x + 15).

∴ x + x + 15 = 195

⇒ 2x = 195 – 15

⇒ 2x = 180

⇒ x = 180/2

⇒ x = 90

Hence, the numbers are 90 and (90 + 15) = 105.

(10) Let the numbers are x, (x + 1) and (x + 2).

∴ x + x + 1 + x + 2 = 54

⇒ 3x + 3 = 54

⇒ 3x = 54 – 3 = 51

⇒ x = 51/3

⇒ x = 17

Hence, the numbers are 17, (17+1) = 18 and (17 + 2) = 19.

(11) Let the three numbers are x, (x + 2) and (x + 4).

∴ x + x + 2 + x + 4 = 63

⇒ 3x + 6 = 63

⇒ 3x = 63 – 6

⇒ x = 57/3

⇒ x = 19

Hence, the numbers are 19, (19 + 2) = 21 and (19 + 4) = 23.

(12) Given, 2x = 86

⇒ x = 86/2

⇒ x = 43

The man sped = Rs 6

Hence, the total money = Rs (43 + 6) = Rs 49.

(13) Let the age of the son be x years and his father’s age is 4x years.

After 20 years son’s age = (x + 20) years

And father’s age = 4x + 20

But given after 20 years father’s age will be 2(x + 20) years.

Then,  4x + 20 = 2(x + 20)

⇒ 4x + 20 = 2x + 40

⇒ 4x – 2x = 40 – 20

⇒ 2x = 20

⇒ x = 20/2 = 10

Hence, the son’s age is 10 years and father’s age is (4 × 10) = 40 years.

(14) Let the number be x.

∴ 3x – 5 = 16

⇒ 3x = 16 + 5 = 21

⇒ x = 21/3

⇒ x = 7

(15) Let the three numbers are x, (x + 1) and (x + 2).

∴ x + x + 1 = 15 + x + 2

⇒ 2x + 1 = x + 17

⇒ 2x – x = 17 – 1

⇒ x = 16

Hence, the numbers are 16, (16 + 1) = 17 and (16 + 2) = 18.

(16) Let the smallest number be x and the larger be x + 7.

∴ 6x + 7 + x = 77

⇒ 7x = 77 – 7

⇒ x = 70/7

⇒ x = 10

Hence, the numbers are (10 +7) = 17 and 10.

(17) Let the breadth be x and length = (x + 5) m.

∴ 2 (x + x + 5) = 142

⇒ 2(2x + 5) = 142

⇒ 4x + 10 = 142

⇒ 4x = 142 – 10

⇒ x = 132/4

⇒ x = 33

Hence, the breadth is 33 m and length (33 + 50 = 38 m.

(19) Let the age of the son be x years and his father’s age be 3x years.

After 12 years son’s age is = (x + 12) years.

And his father’s age is = (3x + 12) years.

But given, after 12 years his father’s age will be = 2(x + 12) years.

∴ 3x + 12 = 2x + 24

⇒ 3x – 2x = 24 – 12

⇒ x = 12

Hence, the present age of the son is 12 years and his father’s age is (3 × 12) = 36 years.

(20) Let the number of Rs 5 notes = x and Number Rs 10 = 10(90 – x)

∴ 5x + 10(90 – x) = 500

⇒ 5x + 900 – 10x = 500

⇒ – 5x = 500 – 900 = – 400

⇒ x = 400/5

⇒ x = 80

Hence, the number of Rs 5 notes is 80 and Rs 10 notes = (90 – 80) = 10.

Get next Exercise 13A solutions click here


3 Comments

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  1. Done in simple and understandable method.Excellent

  2. thank u made homework easy

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