7 th 1.2

7th Standard Mathematics — Chapter 1

Exercise 1.2 Solutions

1. Fill in the blanks

1. -44 + -44 = -88
   
   Working: -88 - (-44) = -88 + 44 = -44
2. 30 - 75 = -45
   
   Working: -45 + 75 = 30
3. -30 - (+50) = -80
   
   Working: -80 + 50 = -30

2. Say True or False

1. (-675) - (-400) = -1075
   Answer: False
   Reason: (-675) + 400 = -275, not -1075.
2. 15 - (-18) is the same as 15 + 18
   Answer: True
   Reason: Subtracting a negative number is equivalent to adding its positive counterpart.
3. (-45) - (-8) = (-8) - (-45)
   Answer: False
   Reason: Subtraction of integers is not commutative. LHS = -37, whereas RHS = +37.

3. Find the value of the following

1. -3 - (-4) using number line:
   Start at -3 on the number line. Subtracting -4 means moving 4 units to the right.
   Answer: 1
2. 7 - (-10) using number line:
   Start at 7 on the number line. Subtracting -10 means moving 10 units to the right.
   Answer: 17
3. 35 - (-64)
   = 35 + 64 = 99
4. -200 - (+100)
   = -200 - 100 = -300

4. Word Problem 1

Kabilan was having 10 pencils with him. He gave 2 pencils to Senthil and 3 to Karthik. Next day his father gave him 6 more pencils, from that he gave 8 to his sister. How many pencils are left with him?

Solution:

• Initial pencils = 10
• Pencils given away = -2 - 3 = -5
• Pencils received from father = +6
• Pencils given to sister = -8

Calculation: 10 - 2 - 3 + 6 - 8 = 16 - 13 = 3

Answer: 3 pencils are left with him.

5. Word Problem 2

A lift is on the ground floor. If it goes 5 floors down and then moves up to 10 floors from there. Then in which floor will the lift be?

Solution:

• Ground floor = 0
• 5 floors down = -5
• 10 floors up = +10

Calculation: 0 - 5 + 10 = +5

Answer: The lift will be on the 5th floor (above the ground floor).

6. Word Problem 3

When Kala woke up, her body temperature was 102°F. She took medicine for fever. After 2 hours it was 2°F lower. What was her temperature then?

Solution:

• Initial body temperature = 102°F
• Decrease in temperature = 2°F

Calculation: 102°F - 2°F = 100°F

Answer: Her temperature was 100°F.

7. Number Finding

What number should be added to (-17) to get (-19)?

Solution:

Let the required number be x.

(-17) + x = -19

x = -19 - (-17)

x = -19 + 17 = -2

Answer: -2 should be added.

8. Justification Problem

A student was asked to subtract (-12) from (-47). He got -30. Is he correct? Justify.

Solution:

Subtracting (-12) from (-47):

= (-47) - (-12)

= -47 + 12

= -35

Answer: No, he is not correct.

Justification: The correct answer is -35. The student likely added -12 instead of subtracting it, or made a sign error during addition (-47 + 12 ≠ -30).