MENTAL MATHS CLASS 7

 

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1. −102 − (−92)
   = −102 + 92 = −10

2. 4 − 5 + 6 − 7 − (−4)
   = 4 − 5 + 6 − 7 + 4 = 2

3. 72 and −190
   = 72 + (−190) = −118

4. 1800 + 60 + (−20)
   = 1800 + 60 − 20 = 1840

5. 55 + (−7 × 5) − 12
   = 55 − 35 − 12 = 8

6. Subtract the sum of −1777 and 777 from 4000
   Sum = −1777 + 777 = −1000
   4000 − (−1000) = 5000

7. −502 + 605 + (−120)
   = −502 + 605 − 120 = −17

8. How many integers are greater −12 but less than −7
   Integers: −11, −10, −9, −8 → 4 integers

9. Additive inverse of 375 − (115 + 160)
   375 − (115 + 160) = 375 − 275 = 100
   Additive inverse of 100 = −100

10. (−225) × 550 + 1225 × 550
    = (−225 + 1225) × 550 = 1000 × 550 = 550000

11. Two integers with difference 6 and sum −18
    Let the integers be x and y:
    x + y = −18, x − y = 6 → Solving, x = −6, y = −12
    Answer: −6 and −12

12. 1 − 2 + 3 − 4 + 5 − 6 + ... + 16
    Grouping pairs: (1−2)+(3−4)+(5−6)+...+(15−16) = (−1)+(−1)+... = 8 pairs × (−1) = −8

13. Product of first 3 multiples of 5
    5, 10, 15 → 5 × 10 × 15 = 750 → 750

14. (−125) × (−537) × (−8)
    = Positive × Negative = −537000

15. Odd integers < 100 multiple of 3 & 5 (i.e., 15)
    Multiples of 15 < 100: 15, 45, 75 → all odd → 3

16. Subtract the sum of (−1032) and 878 from −34
    Sum = −1032 + 878 = −154
    −34 − (−154) = −34 + 154 = 120

17. −1999 − 1999 × 99
    = −1999 − (1999 × 99) = −1999 × (1 + 99) = −1999 × 100 = −199900

18. (−3) + (−12) ÷ (−4) − 3 × (−3)
    = −3 + 3 − (−9) = −3 + 3 + 9 = 9

19. 32 − [32 − (32 − (32 − 32))]
    Inner to outer:
    = 32 − [32 − (32 − 0)]
    = 32 − [32 − 32]
    = 32 − 0 = 32

20. Three × (one less than difference of 19 and 5)
    Diff = 14, one less = 13 → 3 × 13 = 39

21. Some number added to −11 = 37
    Number = 48
    48 ÷ (−12) = −4
    −4 × (−8) = 32

22. 90 ÷ (90 ÷ (90 ÷ 3))
    = 90 ÷ (90 ÷ 30) = 90 ÷ 3 = 30

23. 100 − 8000 ÷ 40 + 95
    = 100 − 200 + 95 = −105 + 95 = −10

24. 90 − (−3) × (−3) × (−3)
    = 90 − (−27) = 90 + 27 = 117

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