Page 131 - Start Up Mathematics_7

Example 7: Find: (a) 10% of 640 (b) 35% of 1 kg (c) 1% of 1 hour
10
Solution: (a) 10% of 640 = × 640 = 64
100
35
(b) 35% of 1 kg = × 1,000 g = 350 g
100
1 3
(c) 1% of 1 hour = × 60 min = × 60 s = 36 s
100 5

Example 8: Find the whole quantity if: (a) 30% of it is 600 km. (b) 6% of it is 30 litres.
Solution: Let the whole quantity in each case be q.
(a) 30% of q = 600 km (b) 6% of q = 30 litres
30 6
⇒ × q = 600 km ⇒ × q = 30
100 100
600 × 100 30 × 100
⇒ q = = 2,000 km ⇒ q =
30 6
⇒ q = 500 litres

Example 9: Convert the given percents to decimal fractions and also to fractions in the simplest
form:
(a) 75% (b) 150% (c) 0.05% (d) 15%
75 3 150 3
Solution: (a) 75% = = (b) 150% = =
100 4 100 2
0.05 5 1 15 3
(c) 0.05% = = = (d) 15% = =
100 10,000 2,000 100 20

Example 10: In a class, 20% of the students got distinction, 40% got first division and another
25% got second division. The remaining students got third division. What percentage
of the students got third division?
Solution: We know, Distinction = 20% ; First division = 40% ; Second division = 25%
∴ Third division = 100% – (Distinction + First division + Second division)

= 100% – (20% + 40% + 25%) = 15%
Example 11: Out of 30,000 voters in a constituency, 55% voted. Find the percentage of voters
who did not vote. Also find the number of voters who did not vote.

Solution: Number of voters in the constituency = 30,000
Percentage of voters who voted = 55%

∴ Percentage of voters who did not vote = 100% – 55% = 45%
45
Number of voters who did not vote = 45% of 30,000 = 100 × 30,000 = 13,500
Example 12: Rupal saves ` 1,200 from her salary. If this is 10% of her salary, what is her salary?
Solution: Rupal’s 10% of Salary = ` 1,200
10
i.e., × Salary = ` 1,200
100
100
∴ Salary = ` 1,200 × = ` 12,000
10

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