Year 5 Maths: Mental Calculations with Decimals

Introduction to Decimals

Hello, Year 5! Today, we’re going to learn about decimals and how to do mental calculations with them. Decimals are numbers that are not whole. They are used to show parts of a whole. For example, 0.5 means half of something, and 0.25 means a quarter.

What are Decimals?

Decimals are written with a point called a “decimal point.” Here are some examples:

• 0.1 (one-tenth)
• 0.25 (twenty-five hundredths)
• 0.75 (seventy-five hundredths)

Key Rules for Mental Calculations with Decimals

1. Align the Decimal Points: When adding or subtracting decimals, make sure to line up the decimal points. This helps you keep the numbers in the correct place.Example: 2.5 + 1.3 ------ 3.8
2. Counting Decimal Places: When multiplying or dividing by 10, 100, or 1000, you can quickly move the decimal point to the right or left.
   • Multiply by 10: Move the decimal one place to the right.
   • Divide by 10: Move the decimal one place to the left.
   Example:
   • Multiply: $$0.5 \times 10 = 5.0$$ (or just 5)
   • Divide: $$5.0 \div 10 = 0.5$$
3. Rounding Decimals: Rounding can make mental calculations easier. If a decimal is 0.5 or higher, round up. If it’s less than 0.5, round down.Example:
   • Rounding 0.7 to the nearest whole number gives you 1.
   • Rounding 0.4 gives you 0.

Tips and Tricks

• Practice in Your Head: Try to do simple calculations in your head without writing them down. This can help you become quicker and more confident!
• Use Estimation: Estimate your answers by rounding decimals to make them easier to work with.
  • For example, if you’re adding 2.7 and 3.4, you can round them to 3 and 3, making it easier to see that the answer is about 7.

Questions for You!

Now, let’s practice what we’ve learned. Here are some questions for you to try.

Easy Level (1-20)

1. 0.5 + 0.3 = ?
2. 1.2 + 0.4 = ?
3. 2.0 – 1.5 = ?
4. 0.7 + 0.2 = ?
5. 3.1 – 2.1 = ?
6. 0.25 + 0.75 = ?
7. 1.5 + 2.5 = ?
8. 4.0 – 1.0 = ?
9. 0.6 + 0.4 = ?
10. 2.2 – 1.0 = ?
11. 1.0 + 0.5 = ?
12. 0.9 – 0.3 = ?
13. 2.5 + 0.5 = ?
14. 3.3 – 0.3 = ?
15. 1.4 + 0.6 = ?
16. 2.5 – 0.5 = ?
17. 0.8 + 0.1 = ?
18. 1.3 – 0.3 = ?
19. 0.4 + 0.6 = ?
20. 3.0 – 2.0 = ?

Medium Level (21-40)

21. 2.25 + 1.75 = ?
22. 4.2 – 2.6 = ?
23. 0.5 + 0.8 = ?
24. 3.6 + 2.4 = ?
25. 4.5 – 1.5 = ?
26. 5.0 + 2.0 = ?
27. 3.3 + 1.7 = ?
28. 2.8 – 0.8 = ?
29. 1.9 + 0.1 = ?
30. 2.4 + 2.6 = ?
31. 5.5 – 0.5 = ?
32. 3.7 + 1.3 = ?
33. 4.0 – 2.1 = ?
34. 0.75 + 0.25 = ?
35. 3.2 + 0.8 = ?
36. 2.5 – 1.5 = ?
37. 6.0 – 3.0 = ?
38. 0.6 + 0.2 = ?
39. 1.2 + 1.8 = ?
40. 3.5 – 1.5 = ?

Hard Level (41-60)

41. 1.25 + 0.75 = ?
42. 3.45 + 2.55 = ?
43. 5.6 – 2.3 = ?
44. 4.75 + 0.25 = ?
45. 3.9 – 1.5 = ?
46. 2.1 + 2.9 = ?
47. 6.25 – 2.25 = ?
48. 4.5 + 3.5 = ?
49. 2.75 – 1.25 = ?
50. 5.1 + 0.9 = ?
51. 3.15 + 1.85 = ?
52. 7.0 – 4.0 = ?
53. 0.99 + 0.01 = ?
54. 8.5 – 3.5 = ?
55. 1.45 + 2.15 = ?
56. 4.6 – 1.6 = ?
57. 9.9 – 0.9 = ?
58. 3.75 + 1.25 = ?
59. 6.0 – 2.5 = ?
60. 2.33 + 0.67 = ?

Answers

Easy Level Answers

1. 0.8
2. 1.6
3. 0.5
4. 0.9
5. 1.0
6. 1.0
7. 4.0
8. 3.0
9. 1.0
10. 1.2
11. 1.5
12. 0.6
13. 3.0
14. 3.0
15. 2.0
16. 2.0
17. 0.9
18. 1.0
19. 1.0
20. 1.0

Medium Level Answers

21. 4.0
22. 1.6
23. 1.3
24. 6.0
25. 3.0
26. 7.0
27. 5.0
28. 2.0
29. 2.0
30. 5.0
31. 5.0
32. 5.0
33. 1.9
34. 1.0
35. 4.0
36. 1.0
37. 3.0
38. 0.8
39. 3.0
40. 2.0

Hard Level Answers

41. 2.0
42. 6.0
43. 3.3
44. 5.0
45. 2.4
46. 5.0
47. 4.0
48. 8.0
49. 1.5
50. 6.0
51. 5.0
52. 3.0
53. 1.0
54. 5.0
55. 3.6
56. 3.0
57. 9.0
58. 5.0
59. 3.5
60. 3.0

Conclusion

Great job, Year 5! Keep practicing your mental calculations with decimals. Remember to align your decimal points, move them carefully when multiplying or dividing, and use rounding when you need to. You’re doing fantastic!