Year 3 Teaching Sequence 11 - Multiplication and division (including commutativity, grouping and finding a remainder) (four days) Prerequisites: Know by heart multiplication facts for 2, 5, and 10 times tables (see oral and mental starter banks 10 and 11) Begin to know by heart multiplication facts for the 3 times table (see teaching sequence 10 and oral and mental starter bank 11) Count on in 3s and 4s (see teaching sequence 10 and oral and mental starter bank 11) Understand grouping as one model of division (see Year 2 Summer teaching sequence 11 and Year 3 Autumn teaching sequence 10) Overview of progression: Children revise the concept of commutativity (a x b b x a) and use this to solve multiplication of single-digit numbers choosing which order will help them to make use of their multiplication facts. They use grouping to divide numbers by 2, 3, 4, 5 and 10 including those which leave a remainder and investigate which numbers between 10 and 20 leave remainders when divided by 2, 3, 5 or 10. They use division and their knowledge of multiplication facts to find mystery numbers in multiplication sentences. They write division sentences to go with multiplication sentences. Note that multiplication sentences can be read in two ways; 4 × 5 can be read as four fives (4 lots of 5) or alternatively as 4 multiplied by 5, i.e. 4, five times (5 lots of 4). Children need to be aware of the different terminology used (lots of, times, multiplied by), but more importantly understand that multiplication is commutative and so the answer is the same which ever way round we write a multiplication. Thus 4 lots of 5 is equivalent to 5 lots of 4. Understanding commutativity helps chn to multiply efficiently. Note that the grouping model of division is used in this sequence to relate multiplication and division. It is not useful to use a sharing model. There is a more natural link between grouping and multiplication facts (e.g. four 10s are 40, how many 10s in 40?). For this reason it is important that the ÷ sign is not read as ‘sharing’, which would confuse the two models. Watch out for children who try to share between groups rather than using grouping, as once learnt this will be more efficient, easier to visualise and needs to be learned for when they divide larger numbers too cumbersome to share. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y3 Maths TS11 – Aut – 4days Objectives: Multiply single digit numbers by 2, 3, 4, 5 and 10, and divide two-digit numbers by the same (answers not greater than 10) Understand how multiplication is commutative Understand that division can leave a remainder (initially as ‘some left over’) Whole class Group activities Paired/indiv practice Resources Write 20 + 54 on the board. How would you work this out? Which number would you start with? Why? Would the answer still be the same? Explain that this is because addition works in any order and that multiplication is the same. Write 5 × 7 on the board. Explain that this can be read as ‘five lots of seven’, or ‘five multiplied by seven’ (seven lots of five). Which way would you work this one out? Five lots of seven or seven lots of five? We haven’t learned our sevens yet so perhaps seven lots of five might be easier! Re-write the calculation as 7 x 5 and read as seven lots of five. This is how we have decided to do this! Draw seven jumps of five above a 0-100 beaded line and five jumps of seven below it, to show that they come to the same number. Show an array of seven rows of five: Group of 4-5 children Write 30 in the middle of the flipchart. What multiplication facts do we know with an answer of 30? How many tens are in 30? If we know that there are 3 tens in 30, what else do we know? Write 10 × 3 = 30, 3 × 10 = 30, 30 ÷ 10 = 3 and 30 ÷ 10 = 3 in a new bubble joined to 30 in the middle. What other numbers go into 30 without leaving any over? Build up other facts including using 15 × 2, 5 × 6 and other related facts. Discuss double 15 and how we can write this as a multiplication. Easier: Give chn 30 cubes to put into equal groups to help. Write the following multiplications on the board: 4 × 3, 10 × 3, 7 × 5, 2 × 4, 9 × 10, 7 × 2, 5 × 6, 2 × 9, 8 × 5, 5 × 4, 3 × 6. Chn work through them to practise multiplying single digits together. They write how they decided to find the answer, so for example for 5 × 8 they write 8 lots of 5 if they choose to count on in 5s or use their tables facts for 5 rather than counting on in 8s. Chn use a 0-100 beaded line (see resources) to help them with any for which they don’t yet know the multiplication facts (particularly multiples of 4). Easier: Chn will probably need to use a 0-100 beaded line to support counting on more frequently. Harder: They also choose one multiplication to show both ways, drawing two empty number lines. Arrays as opposite A large 0-100 beaded number line Cubes 0-100 beaded lines (see resources) Discuss how we can find the total by counting in fives down the rows, or else in sevens across the columns. Rotate the array to show five rows of seven, and discuss how we can count in © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y3 Maths TS11 – Aut – 4days fives or seven to find the same total. Write 5 × 3 on the board. Would you prefer to work out five threes or three fives? Draw hops to show three jumps of five and five jumps of three. Show an array of three rows or five and discuss as before. Write 2 × 8 on the board. How would you work this out? Discuss how we could count in twos, or use our knowledge of doubles, in this case double 8, and so we are thinking of two groups of eight. Write 4 × 6. We don’t yet know our 4 or 6 times table, would you prefer to count on in 4s or 6s? Show recording hops of 4 on a 0-100 beaded line to help. Write 20 ÷ 5 on the board. What does this mean? Read it both as ‘twenty divided by five’, and ‘we have 20, how many fives?’ Write ? x 5 = 20. How many fives are in 20? Write 21 ÷ 5. What if it was 21 instead of 20? What would happen? Launch the ITP Grouping, and choose 21 ÷ 5 as the calculation. Click on 21 to show 21 objects, and then click on five to show one group. Can we make another five? Repeat clicking on fives until there is only one left. What's happened? Write 21 ÷ 5 = 4 and 1 left over. Say that we call this one left over a remainder. Group of 4-5 children What remainders do you think are possible when dividing by five? Take chn’s suggestions. Take 11 cubes and divide them into groups of five. What happens? What do you think will happen if you divided 12 into groups of five? Ask chn to try dividing 13, 14 and 15 into groups of five. What happened when you divided 14 by 5? What happened when you added another cube? Discuss how this made another group of five, and so remainders of 1, 2, 3 and 4 are possible when dividing by 5. What remainders do you think are possible when dividing by 3? Use cubes and test out your ideas. Ask chn to work in pairs to investigate which numbers less than 50 will leave a remainder of 1 when divided by 5. They record the divisions, e.g. 11 ÷ 5 = 2 and 1 left over. They can use 0-50 beaded lines to help where necessary. Easier: Chn use cubes to support the division. Harder: Chn also find at least four numbers less than 30 will leave a reminder of 1 when divided by 3. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. ITP Grouping 0-50 beaded lines Cubes Y3 Maths TS11 – Aut – 4days Harder: Also ask what remainders are possible when dividing by 4 and 10. Can they generalise the biggest possible size reminder when dividing by any number? What do you think would happen if we had 22 divided by 5? And 23? Write 21 ÷ 5 on the board. What does this mean? Read it as 21 divided into groups of 5. How many groups will we have? And left over? Show this on the Grouping ITP to confirm. What do you think will happen if we divide 21 into groups of 3? Use the ITPs to confirm that there will not be a remainder. And what if we divide 21 into groups of 10? And if divide 21 into groups of 2? Agree that there will be one left over each time and show this on the Grouping ITP. Repeat for 22 ÷ 5, 22 ÷ 2, 22 ÷ 3 and 22 ÷ 10. Group of 4-5 children Write 23 in the middle of the flipchart. Do you think we would get any left over if we divided 23 by 2? Is 23 a multiple of 2? How do you know? Ask chn to check this by working in pairs to put 23 into group of 2. Write 23 ÷ 2 = 11 and 1 left over. Repeat, this time dividing 23 by 3, then 4, 5 and 10, building up a fact web. Ask chn to work in pairs to investigate which numbers from 10 to 20 will leave a remainder when divided by 2, 3, 5 and 10. They record their results on a table (see resources) and discuss why they think some numbers leave remainders. They use 0-20 beaded lines to support their calculations. Easier: Chn use cubes to support the division. Harder: Chn also find the next two numbers which will leave some left over when divided by 2, 3, 5 and 10. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. ITP Grouping Activity sheet of table on which to record results of division investigation 0-20 beaded lines Cubes Y3 Maths TS11 – Aut – 4days 23 ÷ 2 = 11 and 1 left over 23 ÷ 10 = 2 and 3 left over 23 ÷ 5 = 5 and 3 left over 23 cubes 23 ÷ 3 = 7 and 2 left over 23 ÷ 10 = 2 and 3 left over 23 always leaves some over when divided by 2, 3, 4, 5 or 10. Why do you think this is? Discuss how it is not in any of their times tables, and how actually the only numbers that go into it without leaving some left over are 1 and 23! This is a very special number; it is called a prime number. Easier: Use 13 cubes. Harder: Also challenge chn to think of a number that wouldn't leave any left over when divided by 2, 5 or 10. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y3 Maths TS11 – Aut – 4days Write 5 × 3 = 15 on a card, and use a ‘slidy’ box’ to cover up the 3. 5 × - = 15 What number do you think might be hiding? Why do you think that? We could read this as five lots of something equals fifteen, or five multiplied by something equals fifteen. How many fives are in fifteen? What division number sentence could we write? Reveal the mystery number. Show the following: 364 × 5 = 20 What number do you think is hiding? Why do you think that? We can read this as something multiplied by 5 equals 20, or five lots of something equals 20. How many 5s are in 20? What division number sentence could we write? Reveal the mystery number. Repeat with slidy box cards for other multiplication facts e.g. 5 × □ = 15, □× 4 = 40, □× 5 = 40, 4 × □= 12, 3 × □= 21. Group of 4-5 children Record □× □= 40 on the flipchart. What do you think the mystery numbers might be? Can you think of a number that goes into 40 exactly? Take suggestions. And how many 10s are in 40? What two multiplication sentences could we write? Record 10 × 4 = 40 and 4 × 10 = 40. We know that ten 4s are 40, but this also means four 10s are 40, so 4 goes into 40 too. What division sentences could we write to go with these? Record 40 ÷ 4 = 10 and 40 ÷10 = 4. Repeat with other suggestions until you have families of four facts including those derived from 20 × 2 and 8 × 5. Repeat with 24. Easier: Use □ × □ = 20 instead, and try and make arrays with a total of 20 counters/cubes. Rpt with 30. Chn work in pairs to each fill in the empty boxes on an activity sheet (see resources). Encourage them to use multiplication to check their answers, counting in fours, for example, on their fingers if they don’t know the appropriate multiplication fact. Easier: Chn use 0-100 beaded lines to help them to work out the mystery multiplications. Harder: Ask chn to also write division sentences to go with at least four of the multiplications. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Slidy box cards as opposite Activity sheet of multiplication sentences with missing numbers (see resources) Y3 Maths TS11 – Aut – 4days