Year 5 Teaching Sequence 4 - Multiplying and dividing two- and three-digit numbers by single-digit numbers (five days) Prerequisites: Know by heart multiplication facts for 2, 3, 4, 5, 6, 9 and 10 times tables, and learn corresponding division facts (see Year 4 Summer teaching sequence 3 and oral and mental starter banks 3 and 4) Use the grid method to multiply two-digit numbers by single digits. Use chunking on the ENL to divide two-digit numbers by single digits, including those leaving a remainder (see Year 4 Summer teaching sequence 9) Overview of progression: Children revise using the grid method to multiply two-digit and single-digit numbers together, and then use this to multiply three-digit numbers by single-digit numbers. Further practice is given, and they are asked to use their experience to approximate first. They are also asked to spot calculations that are easy to do mentally and where the grid method would be less efficient. Children revise using chunking on the empty number line (ENL) to divide two-digit numbers by single-digit numbers including those leaving a remainder, approximating first, and then move on to divide three-digit numbers by singledigit numbers (answers less than 30). They are encouraged to spot the occasional division where quartering (sharing) might be more efficient than chunking (grouping) and also shown how to check division using multiplication. Word problems are given and children are asked to round up or down after their calculations in order to answer the problems. Note that some children might find it helpful to list multiplication facts for multiples of ten of the divisor (e.g. 10, 20, 30 lots of 6 when dividing by 6) and this will become more helpful in later terms when the answers increase to beyond 30. Note that children need to understand that both the grid method and chunking on the ENL are both structured recordings to support what is essentially mental calculation. Watch out for children who are not fluent in multiplying by multiples of ten. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y5 Maths TS4 – Aut – 5days Objectives: Approximate first Multiply multiples of 100 to 900 by a single digit, e.g. 700 x 4 = 4 x 700 = 4 x 7 x 100 = 2800 Use the grid method to multiply three-digit numbers by single-digit numbers Use chunking on the ENL to divide two- and three-digit numbers by single-digit numbers, including those leaving a remainder Decide whether to group or share (including halving and quartering) to solve division Decide whether to round up or down after division Whole class Group activities Paired/indiv practice Resources How did we show how we worked out 34 × 3 using brackets? And how did we use the grid method to show this last year? Ask chn to revise using the grid method to work out 45 × 5, 76 × 3 and 58 × 6. Write 134 × 3 on the board. Discuss with your partner how you might work this out. Take feedback and draw out that the same partitioning strategy can be used. Show how the grid method can be used to record the steps: × 100 30 4 Group of 4-5 children Write the following multiplications on the flipchart, and explain that four of them are correct and four are wrong. 225 × 6 = 1236 523 × 4 = 2092 446 × 5 = 2230 524 × 3 = 872 330 × 8 = 2424 625 × 4 = 2500 712 × 9 = 6408 254 × 5 = 10270 Ask chn to discuss which ones might be wrong and why, for example looking at units’ digits and approximations. Easier: Include more examples of multiplying by 2, 3, 4 and 5. Harder: Include more examples of multiplying by 6, 8 and 9. Chn practise using the gird method to support multiplying three-digit numbers by singledigit numbers (see Activity sheet in resources). Easier: Use a slightly easier activity sheet giving easier products to add together in the first instance (see resources). Harder: Ask chn to work through every other one, and make up their own three-digit numbers to multiply by single digits. Activity sheets (see resources) 3 300 90 12 402 What if it were 234 × 3? Work in pairs to use the grid method to show this on your whiteboards. Repeat with 145 × 5, 276 × 3 and 158 × 6. Repeat with 345 × 5. What happens now? Discuss how the product of 300 and 5 makes a four-digit number, i.e. 1500. Repeat with 576 × 3 and 358 × 6. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y5 Maths TS4 – Aut – 5days Whole class Group activities Write the following multiplications on the board: 6 x 376, 319 x 5, 3 x 482, 407 × 4 Which of these do you think will have the biggest answer? To the nearest hundred, what will the answer be? How do you know? Will it be more or less than 1800? Which will have the smallest answer? To the nearest hundred, what will the answer be? Will it be more or less than 1500? Write approximations for the other two on your whiteboards. Would you use the grid method for all of these? How would you work out 407 × 4? Watch out for multiplications that can be done mentally without using a written method! Work out all four answers, and point out that with multiplication, approximations are often further away from the exact answer than for approximations made for subtractions and additions for example. Write the following multiplications on the board: 610 × 4, 7 x 289, 3 x 783, 6 x 551 Write the calculations in order on your whiteboards with your approximations at the side. Take feedback. Discuss how 551 is roughly half way between 500 and 600, so the answer will be roughly half way between 3000 and 4000, i.e. around 3500. Would you use the grid method for all four calculations? How might you record your steps when working out 610 × 4? Discuss writing 2400 + 40 = 2440, and also the option of using brackets: 610 × 4 = (600 × 4) + ( 10 × 4) = 2400 + 40 = 2440 Group of 4-5 children Write the following multiplications and answers (in the wrong order) and ask chn to match them up. 314 × 5 2168 271 × 8 1656 428 × 4 1570 624 × 3 4908 792 × 5 4689 818 × 6 1872 521 × 9 3960 What clues can we use? Discuss the units’ digits and approximations. Which multiplications will give an answer with 0 in the units’ place? Which multiplication could give an 8 in the units place? Any others? So which is which? Why do you think that? Split the calculations between the group to check. Easier: Include more examples of multiplying by 2, 3, 4 and 5. Harder: Include more examples of multiplying by 6, 8 and 9. Paired/indiv practice Resources Chn practise making approximations before choosing to use the grid method or a jotting to support mental calculation when multiplying three-digit numbers by single-digit numbers. Easier: Use the main activity sheet from yesterday, but also make approximations. Harder: Ask chn to work through every other calculation and then to come up with their own three-digit by single-digit multiplications with approximations of 1200, 1600, 1800, 2000 and 2500. © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Activity sheets (see resources) Y5 Maths TS4 – Aut – 5days Whole class Group activities Paired/indiv practice Resources Write 92 ÷ 6 on the board and sketch a line from 0 to 92. How many 6s do you think might be in 92? More than 10? More than 20? So if there are more than 10, we can draw one big jump of 10 lots of 6, rather than lots of little hops of 6. Draw and label one jump from 0 to 60, also labelling 60. How much is left? And how many 6s are in 32? Draw a jump to show five lots of 6, also labelling 90. And how much is left now? Can we get another 6 in that space? No, so this is our remainder. Label this section r 2. So how many 6s are in 92? Write 92 ÷ 6 = 15 r 2. Our estimating that there were between 10 and 20 lots of 6 in 92 helped us. Remind chn that this method of division is called ‘chunking’ as we don’t count the 6s individually, but find how many 6s are in larger ‘chunks’ of the number we are dividing. Write 76 ÷ 3 on the board. How many 3s are in 76? More than 10? More than 20? More than 30? Record: 10 lots of 3 = 30 20 lots of 3 = 60 30 lots of 3 = 90 Agree that 20 lots of 3 is 60 and 30 lots of 3 is 90 and so the answer is between 20 and 30, probably about half way between. Draw a large jump of 20 lots of 3, labelling it and the point where it lands. How much is left? And how many 3s are in 16? Draw a jump to show 3 lots of 5 landing on 75, and label the remainder. So how many 3s are in 76? Record 76 ÷ 3 = 25 r 1. So the answer was roughly half way between 20 and 30. Group of 4-5 children Write 92 ÷ 6 on the flipchart. We drew an empty number line to help us to find how many 6s are in 92. We split 92 into ‘chunks’. First we found how many 6s were in 60, and then in the remaining 32. Record this as: 92 ÷ 6 = (60 ÷ 6) + (32 ÷ 6) = 10 + 5 r 2 = 15 r 2 Ask chn to discuss in pairs how we could use this method of recording to show 76 ÷ 3. Take feedback and repeat with 97 ÷ 4. How could we check our answer? Draw out using multiplication, and show this using brackets: 97 ÷ 4 = 24 r1 (24 × 4) + 1 = (20 × 4) + (4 × 4) + 1 = 80 + 16 + 1 = 97 Ask chn to discuss what chunks they would split 85 into when dividing by 3, 4, 5 and 6. Help them to show this using brackets to ‘chunk’ the 85 in appropriate ways. Chn practise using chunking, recording their approximations first, e.g. between 20 and 25 (see Activity sheet in resources). Easier: Encourage chn to record the multiplication facts for multiples of 10 as for 76 ÷ 3 in the whole class teaching. Chn’s estimate might be ‘between 20 and 30’ rather than ‘between ’25 and 30’ at this stage. Harder: Chn work through every other example, and then write their own divisions by 3, 4 and 5 with answers in the following ranges: Between 15 and 20, between 20 and 25, between 25 and 30, between 30 and 35. Activity sheet of chunking practice (see resources) © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y5 Maths TS4 – Aut – 5days Repeat with 97 ÷ 4. Do you think the answer will be closer to 20 or 30? So the answer is between 20 and 25. Stress how this estimate means that we draw fewer jumps and the chunking is more efficient. Easier: Begin with 39 ÷ 3, 43 ÷ 3, 48 ÷ 4, 57 ÷ 4, then discuss how to split 65 when dividing by 3, 4, 5 and 6. Write 88 ÷ 4 on the board. Ask chn to discuss in pairs how they might work this out. Explain that we could use chunking. Ask chn to draw an ENL on their whiteboards to show this. Remind chn that chunking uses the grouping model of division, but we could also use sharing to solve this division, thinking of sharing 88 between four equal groups rather than thinking of how many 4s might be in 88. Remind chn of the link between sharing between 4 and finding a quarter. How would you find a quarter of 88? Draw out halving and halving again to arrive at 22. Ask chn to vote (now that they have seen both ways) for the way that they would prefer for this particular division. Write 64 ÷ 4, 58 ÷ 4, 76 ÷ 4 and 93 ÷ 4 and ask chn to discuss in pairs how they would solve each, using grouping (chunking) or sharing (finding a quarter). Take feedback. It is likely that chn are happy to use sharing for 64 as both digits are even, both 60 and 4 can be halved twice and give whole number answers. When dividing two-digit numbers by single-digit numbers using chunking watch out for the occasional division where it might be easier and quicker to use sharing, finding a quarter. Group of 4-5 children Write the following division on the flipchart: 127 ÷ 6, 119 ÷ 4, 123 ÷ 5, 128 ÷ 7 And ask chn to discuss in pairs which might have the greatest answer and which might have the smallest. Take feedback, drawing out chn’s reasoning, e.g. thirty 4s are 120 so 119 ÷ 4 will be just under 30. Ask chn to then write the divisions in order from the smallest answer to the greatest. Take feedback, again drawing out chn’s reasoning. Write the following divisions on the flipchart: 48 ÷ 4, 96 ÷ 2, 96 ÷ 8, 24 ÷ 8. Two of these divisions have the same answer, which two do you think they are and why? Let chn discuss this for a while, before drawing out that the first and last are the same. Discuss how the 48 is doubled to make 96, and so the group size is also doubled to keep Chn have further practice using chunking, again recording approximations but also watching out for the occasional division that might be easier solved using quartering. Easier: Slightly easier Activity sheet (smaller answers) see resources). Harder: Slightly harder Activity sheet (see resources, also challenged to make up their own divisions by 6, 7, 8 and 9 with answers of between 20 and 30). © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Activity sheets of chunking practice (see resources) Y5 Maths TS4 – Aut – 5days Display the following problem: A hotel is preparing for a wedding. There are 124 guests who will sit at tables of eight, how many tables are needed? Discuss in pairs what calculation is needed to solve this problem and work together on your whiteboards to solve it. Take feedback. What calculation did you do? And what was the numerical answer. And so what is the answer to the problem? Do we need to round the answer up or down? What do you think the hotel will probably do? Discuss how 124 ÷ 8 = 15 r4, and so 16 tables will be needed, rather than 15 as 4 people would have nowhere to sit. Discuss how the hotel will probably put out two tables for six rather than just the number of groups the same. (96 ÷ 2 will have a much bigger answer where as 24 ÷ 8 has a much smaller answer.) Knowing this, how might you work out 144 ÷ 8? Agree that you could work out 72 ÷ 4, or even 36 ÷ 2. Easier: Ask chn to order the following calculation: 123 ÷ 3, 123 ÷ 4, 123 ÷ 5. Draw out that dividing by a larger number will give a smaller answer as each group is larger, and so there will be fewer of them. Then order the calculations as above. Harder: Ask chn also how they might work out 248 ÷ 16 by finding equivalent calculations. Group of 4-5 children We divided 126 by 8 and got an answer of 15 r6. How could we check that this is correct? Draw out using multiplication. What multiplication would we need to do? Together work out 15 × 8: = (10 × 8) + (5 × 8) = 80 + 40 = 120 But this is 120 not 126? Discuss how we need to add the remainder of 6 on, and so would get 126. Ask chn to work in pairs to check Chn work through problems, finding solutions and deciding whether answers to the required calculations need rounding up or down (see resources). Easier: Some chn may need to sketch diagrams to help them to understand the problem, but still encourage them to work out what calculation is needed. Harder: Ask chn to write ‘up’ or ‘down’ by the side of each problem, and choose four to work out. Then set them the following challenge: © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Activity sheet of word problems (see resources) Y5 Maths TS4 – Aut – 5days one table of four. Display the following problem: A packer for an internet company is gift wrapping remote control sand buggies. It takes 3m of paper to wrap each sand buggy. There is 86m of wrap left, how many buggies can he wrap before he needs to go to the warehouse to get more paper? Ask chn to discuss this in pairs, work out the answer to the calculation and then to the problem. Do you need to round up or down? the following: 137 ÷ 6 = 23 r 5 96 ÷ 7 = 23 r 5 113 ÷ 4 = 28 r 3 172 ÷ 8 = 22 r 4 Easier: Check: 87 ÷ 6 = 13 r 4 96 ÷ 7 = 23 r 5 95 ÷ 4 = 22 r 3 92 ÷ 8 = 11 r 2 the wedding cake has four tiers. The top tier is going to the bride and groom. The other three are to be shared between the guests. The biggest tier is twice the volume of the next biggest tier, which has twice the volume of the next tier. How many pieces do you think each tier should be cut into so that all guests get a piece? © Original teaching sequence copyright Hamilton Trust, who give permission for it to be adapted as wished by individual users. Y5 Maths TS4 – Aut – 5days