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Find the fourth proportional to (i) 3, 12, 15 (ii) 1/3, 1/4, 1/5 (iii) 1.5, 2.5, 4.5 (iv) 9.6 kg, 7.2 kg, 28.8 kg

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This is a basic question from ML aggarwal book of class 10th, chapter 7, ratio and proportion., ICSE board

In this question we have been given three numbers, and we have to find the fourth proportional to them.

Ques2, exercise 7.2

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1. Solution:

(i) 3, 12, 15

Consider x as the fourth proportional to 3, 12 and 15

3: 12 :: 15: x

We can write it as

3 Ã— x = 12 Ã— 15

So we get

x = (12 Ã— 15)/ 3

x = 60

(ii) 1/3, 1/4, 1/5

Consider x as the fourth proportional to 1/3, 1/4 and 1/5

1/3: 1/4:: 1/5: x

We can write it as

1/3 Ã— x = 1/4 Ã— 1/5

So we get

x = 1/4 Ã— 1/5 Ã— 3/1

x = 3/20

(iii) 1.5, 2.5, 4.5

Consider x as the fourth proportional to 1,5, 2.5 and 4.5

1.5: 2.5 :: 4.5: x

We can write it as

1.5 Ã— x = 2.5 Ã— 4.5

So we get

x = (2.5 Ã— 4.5)/ 1.5

x = 7.5

(iv) 9.6 kg, 7.2 kg, 28.8 kg

Consider x as the fourth proportional to 9.6, 7.2 and 28.8

9.6: 7.2 :: 28.8: x

We can write it as

9.6 Ã— x = 7.2 Ã— 28.8

So we get

x = (7.2 Ã— 28.8)/ 9.6

x = 21.6

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