T3 Addition & Subtraction Pt 3

Lesson One

Task One

Learning content: use mental and written strategies for adding and subtracting two, three, and four digit numbers

Have a look at the picture below and work out how many handprints there are of each colour.

Now use this information to calculate the number of handprints altogether (without counting them by ones or twos). Explain how you worked out the total - what strategy could you use?

How many pairs of hands are there?

How many fingers are there?

Task Two

Have a look at these posters and remind yourself how you can use a number line to add and subtract numbers using the jump strategy.

You can use this strategy for 2 digit, 3 digit and 4 digit numbers.

When doing subtraction, REMEMBER that a number line has the smallest number at the left end, and the larger numbers towards the right end. Also remember that subtraction always starts with a large number from which the other numbers are subtracted.

Complete a worksheet given you by your teacher to practise using the jump strategy.

Revision Jump strategy T3.pdf

How might you change the jumps you make when adding or subtracting 3 or 4 digit numbers?

Draw your own poster showing how you could use this strategy for 3 and 4 digit numbers and give your own examples of 3 digit addition and 4 digit addition using the jump strategy.

AND

In your Maths scrapbook, draw and use your own number line to prove whether this number sentence is True or False

572 - 355 = 223

Task Three

Lesson Two

Task One

Last term we looked at a couple of different strategies to use with adding - finding tens (or 20s) and the split strategy. Revise both of these strategies, and have a go at these worksheets.

Friends to 10.pdf

Split strat worksheet.pdf

Task Two

Learning content: use mental and written strategies for adding and subtracting two, three, and four digit numbers

Copy this chart onto a piece of paper and show a different way of solving this addition problem in each box. If you know more than four ways, add extra ways on the back of the sheet, or divide the boxes in half!

Addition strategy Thinkboard L2.pdf

Task Three

Lesson Three

Task One

Learning content: use mental and written strategies for adding and subtracting two, three, and four digit numbers

Last term we had a look at what you needed to add on to a number to make it add up to 10. Today we are going to spend some more time looking at how "friends of 10", and adding numbers on to make the next number that ends with 'zero' is another strategy we can use for addition.

Watch this video for an introduction.

Task Two

Have a look at this image and see if you can understand how it is using "friends of 10" to make another way of adding different numbers.

Task Three

Now have a go at the worksheets.

Bridging to 10.pdf

Bridge to 10.pdf

Task Four

Lesson Four

Activity 1

Counting challenge:

Give yourself 20 seconds to:

count up by 10s from 63
count up by 10s from 178
count down by 10s from 92
count down by 10s from 347

Counting by 10s is very useful when we add and subtract. We use it when we use the Jump Strategy. But today we are going to revise another strategy where counting by 10s makes it really easy to add or subtract numbers.

If you need to add on 9 to a number, it is a lot easier to add on 10 and then take away the extra 1. Click through the slides below:

Activity 2

Use this sheet as a quick revision of the Compensation strategy, then have a go at the worksheet.

Compensation strategy.pdf

Activity 3

See if you can use the same strategy to complete these subtraction sentences.

26 - 9 = 35 - 9 = 145 - 9 =

Activity 4

All working for this activity should be written in your scrapbook or on paper.

Yesterday we revised how you can add on 10 and then subtract 1 as a shortcut for adding 9, and how we can do the same for any number ending in 9. For example, 39 is almost 40, so you could add 40, and then subtract 1. This strategy is called the Compensation strategy.

27 + 39 = 27 + 40 = 67 - 1 = 66 So 27 + 39 = 66

Have a go by yourself with these questions:

35 + 9 2. 34 + 19 3. 42 + 19 4. 56 + 29

5. 45 + 29 6. 53 + 39 7. 64 + 49

A similar pattern works for subtraction.

If you need to subtract 9 from a number, you can subtract 10, and then add 1 back on again (since you have taken away one too many!)

46 - 9 46 - 10 = 36 + 1 = 37 So 46 - 9 = 37

Try some for yourself.

83 - 9 2. 67 - 9 3. 54 - 9 4. 77 - 9 5. 58 - 19

Challenge:

Use the compensation strategy to find the answers to these problems. Show your working.

Charlotte was at the corner store lolly shop. She wants to buy 25 red clouds and 39 chocolate raisins. How many lollies will she buy altogether?
Ebony received $72 dollars for Christmas from her mum and $79 from her Aunty. How much money would she have all together?
86 Cows were in the field, however the farmer sold 39. How many cows are left?

Could you use the compensation strategy for adding any other numbers? Explain.

Activity 5

Spend some time working out what the secret number is for each of these 4 challenges.

Lesson Five

Review

What does = mean?

We call it the equals sign, but equals means 'the same as". We need to remember this when we look at number sentences in maths. What is on one side of the = sign must work out to be the same as what is on the other side of the = sign. They may not look the same, but they must be worth the same.

12 is the same as 8 + 4

7+5 is the same as 8+4. So we can say that 7 + 5 = 8 + 4

Or we can say that 16 - 4 is worth the same as 6 + 6 (because they are both equal to 12)

So we can say that 16 - 4 = 6 + 6 even though they don't look the same.

This is important to remember whenever we see the = symbol.

Spend some time looking carefully at this worksheet. You may feel confident enough to work on it by yourself, you may want to work on it with the teacher for the whole time, or you may want to complete the first few questions with the teacher and then work by yourself when you are sure that you understand what you need to do.

Addition & Subtractioni Equality worksheet.pdf

Make up some similar number sentences of your own and get a friend to check them.

Lesson Six

Sometimes we need to be able to work things out quickly in our heads, and we don't have time to split the numbers into their different parts and to add them up. And sometimes we don't need to have an exact answer, sometimes it is good enough just to have a "rough idea" of how many. To do this easily, we sometimes add up or subtract just numbers that end in zero - like 20 or 40 or 70, because they are really easy to add or subtract. When we do this we call it "rounding".

In the next couple of lessons we are going to learn about, and practise, working out roughly how much an answer might be - this is also a good way of checking whether our answers are correct, or totally "stupid"!

Without counting, suggest roughly what number is represented in the top part of these pictures.

The bottom two images show the tens numbers less than and more than what is in the top picture.

In the first illustration, at the bottom we have 20 and 30. The picture above is bigger than 20, and smaller than 30. Just by looking at the picture, which one do you think is closest?

The top picture is closest to 30.

So we say that we can round 27 to 30, because it is closer to 30 than to 20.

Double check the second picture to make sure you can explain your answer.

You could try making your own questions and giving them to a partner.

Another way we can do this is by looking at a number line.

We decide which two numbers are at either end - the benchmark numbers. And then we write them in.

After we have done that, we need to find what the midpoint is between the two numbers.

Then we need to put the 37 on the number line.

The 37 is closer to 40 than to 30, so when we are rounding, 37 rounds up to 40.

Now try finding the midpoint for these.

Now it's your turn. Complete this worksheet.

Number line rounding worksheet.pdf

Follow the pattern to do rounding with 3 digit numbers.

You can also round 3 digit numbers to the nearest ten. For example, 133 is between 130 and 140. The midpoint would be 135. 133 is closer to 130.

Your teacher will put up some task cards around the room. Try and round those numbers to the nearest ten.

Lesson Seven

Task One

Here's a fun way to learn about rounding and why it's useful! Watch this video story.

Task Two

As a class, have a look at these number lines, and write down which numbers you think will round to the number listed. G3 U1 C5.

Task Three

Practise rounding numbers to the nearest 10 by completing at least three workstations.

Task Four

Rounding is a way we can do a quick check to see if our adding is correct.

If I need to add 58 + 33, I can think that 58 rounds to 60, and 33 rounds to 30, and if I add 60 + 30 the answer will be 90. So I would know that if I got an answer of 101 when I add 58 + 33, then I would know that I might have made a mistake.

Use estimating to decide whether these sentences are True or False.

49 + 38 = 77

68 + 61 = 139

72 + 97 = 169

Lesson Eight

Task One

Learning content: rounding numbers to the nearest 100

We can round larger numbers to the nearest hundred. We can also use a number line to round numbers to the nearest hundred.

Think about the number 488.

Draw a number line that starts at 400 and ends at 500.

Show where you would put the number 488.

Is it closer to 400 or 500? The number it is closest to, is the number it rounds to.

Now think about the number 2321. This number is between 2300 and 2400. Click through the slides below to review how this works.

Task Two

Practise rounding to the nearest hundred by completing at least three work stations.

Task Three

Use rounding to estimate the answers to these questions. Then work them out using jump, split or compensation strategy. Remember that you need to think carefully whether you need to add or subtract in order to work out the problem.

Complete this rounding activity which asks you to round the first eight numbers to the nearest ten, and the second eight numbers to the nearest hundred. Be careful not to be tricked.

rounding-around-the-world.pdf

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