Page 18 - Start Up Mathematics_7

Example 19: A certain freezing process requires the room temperature to be lowered from 30°C
at the rate of 5°C every hour. What will be the room temperature 8 hours after the
process begins?

Solution: Temperature to be lowered by 5°C every hour.
∴ Temperature to be lowered in 8 hours = 8 × 5°C = 40°C

Room temperature in the beginning = 30°C
∴ Room temperature after 8 hours = 30°C – 40°C = –10°C

Example 20: In a class test containing 20 questions, 4 marks are awarded for every correct
answer, 1 mark is deducted for every incorrect answer and 0 is given for questions
not attempted. Answer the following questions:

(a) Rohit gets four correct and six incorrect answers. What is his score?
(b) Arushi gets five correct and five incorrect answers. What is her score?

Solution: (a) Rohit’s score = 4 × 4 + 6 × (–1) = 16 – 6 = 10
(b) Arushi’s score = 5 × 4 + 5 × (–1) = 20 – 5 = 15

Example 21: A cement company earns a profit of ` 8 per bag of white cement sold and a loss of
` 5 per bag of grey cement sold.
(a) The company sells 3,000 bags of white cement and 5,000 bags of grey cement
in a month. What is its profit or loss?
(b) What is the number of white cement bags it must sell to have neither profit nor
loss, if the number of grey bags sold is 6,400? (NCERT)

Solution: (a) Profit or loss of the company = ` 3,000 × 8 + ` 5,000 × (–5)
= ` 24,000 – ` 25,000

= – ` 1,000

∴ There is a net loss of ` 1,000.
(b) Let the number of white cement bags sold for neither profit nor loss be n.

Now, 8 × n + (–5) × 6,400 = 0
⇒ 8n = 5 × 6,400
5 × 6,400
⇒ n =
8
⇒ n = 5 × 800 = 4,000
i.e., the company must sell 4,000 bags of white cement for no profit, no loss.

EXERCISE 1.3

1. Multiply the following:
(a) 12 and 7 (b) –8 and 8 (c) 9 and –11

(d) –6 and –5 (e) 0 and –45

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