1.7 Rounding off

Mathematics

Meet bonny and tommy

Educator section

Memorandum

Leaner section

Content

Activity: rounding off [lo 1.8, lo 1.10]

• Get some counters from your teacher. Estimate how many you have. Write down your estimate.

I estimate ____________________________________________________________

• Now group the counters on the desk in tens and units.
• Count them and write: I have ______________________________________
• How was your estimate: too many, too few or just right?

Write: I estimated ________________________________________________

By how many were you out? _____________________________________________

• Is your number closer to 40, closer to 50 or equally far away from both?

Write: My number is ___________________________________________________

• How can you make your number equal to 50?

Write: I can add _______________ ____________ + ____________ = 50

• If you have added 5 or less than 5, 50 is your nearest multiple of ten.
• How can you make your number equal to 40?

Write: I can subtract _______________ : ____________ - ____________ = 40

• If you subtracted 4 or less than 4, then 40 is your nearest multiple of ten.

To round off a number to the nearest ten you must know which multiple of ten is closer to the number. If your number is the same distance from two multiples of ten, we use the greater multiple of ten.

• Look at the numbers and make an * next to the nearer multiple of ten.

• Now help Bonny and Tommy to find the nearer multiple of ten.

Remember: If the units are 5 or more than 5,

we use the greater multiple.

• If the units are 4 or less than 4, we use the smaller multiple.

24: nearer multiple is 20 16: nearer multiple is 20

37: nearer multiple is _____ 52: nearer multiple is _____

15: nearer multiple is _____ 73: nearer multiple is _____

81: nearer multiple is _____ 94: nearer multiple is _____

• Round off to the nearer 10. (This will help you to estimate):

If we round off, it helps us to estimate what the answer should be more quickly.

Remember! This is not an accurate answer!

• Use rounding off and estimate first. Then do an accurate calculation. Write the rounded off number above the number.
• Tommy has 84c in his purse and buys a banana for 69c.

How much money does he have left?

Rounding off: Number sentence: _____________________________________________________________________

Calculate: Number sentence: _____________________________________________________________________

This will help you to estimate your change when you buy something.

• Bonny prefers to do her sums horizontally.

• Tommy prefers to do his sums vertically.

• Let’s try both ways and then decide which is the easier.

Bonny says: Subtraction is a bit different.

• NEVER rename the first number.

Check your answer: 12 + 64 = 76

• Like this! He thought like this:

• Check your answer:

1 2

+64

76

• What do you think, do you want to do your sums like Bonny or like Tommy?

_____________________________________________________________________

Assessment

Learning Outcome 1: The learner will be able to recognise, describe and represent numbers and their relationships, and to count, estimate, calculate and check with competence and confidence in solving problems.

Assessment Standard 1.8: We know this when the learner can perform calculations, using appropriate symbols, to solve problems involving:

1.8.1 addition and subtraction of whole numbers with at least 3 digits;

1.8.2 multiplication of at least whole 2-digit by 1-digit numbers;

1.8.3 division of at least whole 2-digit by 1-digit numbers;

• estimation;

Assessment Standard 1.10: We know this when the learner uses the following techniques:

1.10.1 building up and breaking down numbers;

1.10.2 doubling and halving;

1.10.3 number-lines;

1.10.4 rounding off in tens.