The document contains examples of math word problems involving subtraction of negative numbers. The first example asks for the current bank balance after an overdraft fee and deposit. It shows subtracting a negative overdraft amount and fee from a positive deposit amount. The second example asks for the temperature difference between the optimum temperatures for two types of bacteria. It shows subtracting one negative temperature from an even lower negative temperature to get a positive difference.

1. Class 7 Maths Exercise 1.3
Question 6.
A certain freezing process requires that room
temperature be lowered from 40oC at the rate of
5oC every hour. What will be the room
temperature 10 hours after the process begins?

2. Given: Present room temperature = 40oC
Decreasing the temperature every hour = 5oC
Room temperature after 10 hours
= 40oC + 10 x (–5oC )
= 40oC – 50oC
= – 10oC
Thus, the room temperature after 10 hours is – 10oC after
the process begins.

3. Question 7
In a class test containing 10 questions, 5 marks are
awarded for every correct answer and
(–2) marks are awarded for every incorrect answer
and 0 for questions not attempted.
(i)Mohan gets four correct and six incorrect answers.
What is his score?
(ii)Reshma gets five correct answers and five incorrect
answers, what is her score?
(iii)Heena gets two correct and five incorrect answers
out of seven questions she attempts. What is her
score?

4. (i)Mohan gets four correct and six
incorrect answers. What is his
score?
Marks obtained for four correct questions
= 4 x 5 = 20
Marks obtained for six incorrect questions
= 6 x (–2) = –12
Therefore, total scores of Mohan
= (4 x 5) + [6 x (–2)]
= 20 – 12
= 8
Thus, Mohan gets 8 marks in a class test.

5. (ii)Reshma gets five correct
answers and five incorrect
answers, what is her score?
(ii) Marks obtained for five correct questions
= 5 x 5 = 25
Marks obtained for five incorrect questions
= 5 x (–2) = –10
Therefore, total score of Resham
= 25 + (–10) = 15
Thus, Reshma gets 15 marks in a class test.

6. (iii)Heena gets two correct and five incorrect
answers out of seven questions she attempts.
What is her score?
(iii) Marks obtained for two correct questions
= 2 x 5 = 10
Marks obtained for five incorrect questions
= 5 x (–2) = –10
Therefore, total score of Resham = 10 + (–10) = 0
Thus, Reshma gets 0 marks in a class test.

7. Example 1: Shyak has overdrawn his checking account by
Rs.38. The bank debited him Rs.20 for an overdraft fee. Later he
deposited Rs.150. What is his current balance?
Solution:Given,
Total amount deposited= Rs. 150
Amount overdrawn by Shyak= Rs. 38
⇒ Debit amount = -38
[Debit is represented as negative integer]
Amount charged by bank= Rs. 20
⇒ Debit amount= -20
Total amount debited = (-38) + (-20) = -58
Current balance= Total deposit +Total Debit
⇒150 + (–58) = 92
[Subtract and give the sign of greater number]
Hence, current balance is Rs. 92.

8. Example 2: Anna is a microbiology student. She was doing research on
optimum temperature for the survival of different strains of bacteria. Studies
showed that bacteria ‘X‘need optimum temperature of -31˚C while bacteria
‘Y ‘need optimum temperature of -56˚C. What is the temperature difference?
Solution:Given,
Optimum temperature for bacteria X = -31˚C
Optimum temperature for bacteria Y= -56˚C
Temperature difference= Optimum temperature for bacteria
X – Optimum temperature for bacteria Y
⇒ (-31) – (-56)
⇒ -31 + 56 = 25
Hence, temperature difference is 25˚C.