## RS Aggarwal Class 7 Math Eighth Chapter Ratio and Proportion Exercise 8C Solution

## EXERCISE 8C

### OBJECTIVE QUESTIONS

#### Mark (√) against the correct answer in each of the following:

**(1) If a : b = 3 : 4 and b : c = 8 : 9, then a : c = ?**

Ans: (d)

a : b = (3 × 2) : (4 × 2) = 6 : 8

∴ a : c = 6 : 9 = 2 : 3

**(2) If A : B = 2 : 3 and B : C = 4 : 5, then C : A = ?**

Ans: (a)

A : B = (2 × 4) : (3 × 4) = 8 : 12

B : C = (4 × 3) : (5 × 3) = 12 : 15

∴ C : A = 15 : 8

**(3) If 2A = 3B and 4B = 5C, then A : C = ?**

Ans: (d)

2A = 3B

or, A : C = 15 : 8

**(4) If 15% of A = 20% of B, then A : B = ?**

Ans: (b)

Ans: (a)

**(6) If A : B = 5 : 7 and B : C = 6 : 11, then A : B : C = ?**

Ans: (b)

A : B = 5 : 7 = (5 × 6) : (7 × 6) = 30 : 42

B : C = 6 : 11 = (6 × 7) : (11 × 7) = 42 : 77

A : B : C = 30 : 42 : 77

**(7) If 2A = 3B = 4C, then A : B : C = ?**

Ans: (c)

2A = 3B

**(10) If x : y = 3 : 4, then (7x + 3y) : (7x – 3y) = ?**

Ans: (c)

= 33 : 9 = 11 : 3

**(11) If (3a+5b) : (3a-5b) = 5 : 1, then a : b = ?**

Ans: (c)

**(12) If 7 : x : : 35 : 45, then x = ?**

Ans: (c)

**(13) What number has to be added to each term of 3 : 5 to make the ratio 5 : 6?**

Ans: (b)

Let the number to be added be x. Then,

**(14) Two numbers are in the ratio 3 : 5. If each number is increased by 10, the ratio becomes 5 : 7. The sum of the numbers is **

Ans: (d)

Let the numbers are 3x and 5x.

Therefore, the sum of the numbers = (3 ×5) + (5 ×5) = 40.

**(15) What least number is to be subtracted from each term of the ratio 15 : 19 to make the ratio 3 : 4?**

Ans: (a)

Let the numbers t be subtracted be x. Then,

**(16) If Rs 420 is divided between A and B in the ratio 3 : 4, then A’s share is**

Ans: (a)

Sum of the ratio = 7

**(17) The boys and girls in a school are in the ratio 8 : 5. If the number of girls is 160, what is the total strength of the school?**

Ans: (d)

Let the number of girls is 5x.

∴ 5x = 160

or, x =3 2

Therefore the number of boys is (8 ×32) = 256.

The total strength of the school = 256+160 = 416.

**(18) Which one is greater out of (2 : 3) and (4 : 7)?**

Ans: (a)

LCM of 3 and 7 is 21.

**(19) The third proportional to 9 and 12 is**

Ans: (c)

Let the third proportional is x.

Then, the fourth proportional to 9, 12, 12 is x.

Hence, the third proportional to 9 and 12 is 16.

**(20) The mean proportional between 9 and 16 is**

Ans: (b)

Let the mean proportional between 9 and 16 be x.

Then, 9 : x : : x : 16

**(21) The age of A and B are in the ratio 3 : 8. Six years hence, their ages will be in the ratio 4 : 9. The present age of A is**

Ans: (a)

Let the present age of A and B is 3x and 8x years.

After, (3x+6) and (8x+6) years.

Therefore the present age of A is (3 ×6) = 18 years.