Page 109 - ICSE Math 7

Example 6: Out of 2,500 candidates 8% candidates did not appear for an examination. Only 70%
of the candidates who gave the examination, passed in it with first and second division.
If the ratio of candidates who passed with first and second division is 3 : 7, find the
number of candidates who passed with second division.
Solution: Number of candidates who did not appear for the exam = 8% of 2,500
8
= × 2,500 = 200
100
∴ Number of candidates who gave the exam = 2,500 – 200 = 2,300

Number of candidates who passed with first and second division = 70% of 2,300
70
= × 2,300 = 1,610
100
7
Number of candidates who passed with second division = × 1,610
3 + 7
7
= × 1,610 = 1,127
10
Example 7: Chalk contains calcium, carbon and oxygen in the ratio 10 : 3 : 12.
(a) Find the percentage of carbon in the chalk.
(b) If in a stick of chalk, carbon is 3 g, what is the weight of the chalk stick?


Solution: (a) Percentage of carbon in chalk =  3 × 100 % = 12%


 25 
(b) Suppose weight of chalk stick = x g
∴ 12% of x = 3
12 3 × 100
or × x = 3 ⇒ x = = 25
100 12
Hence weight of chalk stick = 25 g

EXERCISE 8.1

1. Convert the following into percentage.
3 13 17
(a) 0.128 (b) 0.009 (c) 4 (d) 25 (e) 20
2. Convert the following into fractions.
(a) 34% (b) 50% (c) 64% (d) 78% (e) 88%

3. Convert the following into decimals.
(a) 13% (b) 34.5% (c) 69% (d) 6.9% (e) 8%
4. Express each of the following as a ratio in the simplest form.
2
(a) 50% (b) 200% (c) 16 % (d) 16.25%
3
5. Convert the following ratios into percentage.
(a) 4 : 25 (b) 100 : 120 (c) 5 : 125 (d) 7 : 5
6. Convert each part of the ratio into percentage.

(a) 2 : 3 : 5 (b) 2 : 5 : 9 (c) 5 : 8 : 12 (d) 1 : 2 : 7

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