This unit presents a range of strategies for solving multiplication and division problems with multi-digit whole numbers. Students are encouraged to notice the structure of problems, and to anticipate which strategies might be best suited to solving them. This unit builds on the ideas presented in the Multiplication Smorgasbord session in Book 6: Teaching Multiplication and Division.
The New Zealand Curriculum requires students to understand and use a range of mental, written and digital calculation strategies to multiply and divide multi-digit whole numbers. This unit of work is useful for students working at or towards Level Four Stage 7 - Advanced Multiplicative of the Number Framework). Students at this stage partition and recombine numbers to simplify calculations and draw on their knowledge of multiplication facts and related division facts with factors up to ten. Understanding of whole number place value underpins all strategies in this unit.
The key teaching points are:
The learning opportunities in this unit can be differentiated by providing or removing support to students and by varying the task requirements. Ways to differentiate include:
The contexts for this unit include cycling, working at a fruit shop, transporting people to netball, rowing crews, and delivering pamphlets. Other situations of relevance to your students might be used to capitalise on contextual knowledge and increase motivation. For example, fundraising for an event, preparing a class feast, and organising teams for waka ama may provide useful story shells.
The purpose of this session is to explore the range of strategies that your students already use to solve multiplication and division problems. This will enable you to evaluate which strategies need to be focused on in greater depth as well as identifying students in your group as "expert" in particular strategies.
Problem 1 (Copymaster 2):
Vanessa bikes 38 kilometres each day for five days. How many kilometres has she travelled by the end of the five days?
Ask students to work out the answer in their head if they can and record their strategy on paper. Some students may need to use recording to ease the memory load. Students who work out the problem quickly could be extended by being asked to check their calculations with a different strategy. Give the students an appropriate amount of thinking time. Then ask them to share their solutions with their learning partner. The following are possible responses:
Note that the convention is to record the multiplier first so equations should be written as 5 x 38 = .
Rounding and compensating:
5 x 38
38 is rounded to 40 so the problem becomes 5 x 40 = 200, then 10 (5 x 2) is subtracted from the product to get 5 x 38 = 190.
In full the strategy might be written as 5 x 38 = 5 x 40 – 5 x 2.
View how to do this using Video 1.
Proportional adjustment:
5 x 38
Solve instead 10 x 19 using doubling and halving (by doubling 5 and halving 38).
In full the strategy might be written as 5 x 38 = 10 x 19
View how to do this using Video 2.
Place value partitioning:
5 x 38
Solve 30 x 5, add 8 x 5.
The strategy can be written as 5 x 38 = 5 x 30 + 5 x 8 or in working form:
View how to do this using Video 3.
As different strategies arise ask the students to explain why they chose to solve the problem in that way. Accept all the strategies that are elicited at this stage, recording them to reflect upon later in the unit (perhaps in a modelling book, on a digital document, or on a poster). Be aware that some students may elect to add rather than multiply.
For example:
You might like to discuss the efficiency of multiplication versus repeated addition.
To provide similar problems alter the numbers in Problem One (Copymaster 2), such as:
Vanessa bikes 46 kilometres each day for four days. How many kilometres has she travelled by the end of the four days?
These problems could also be altered to reflect relevant learning from other area of the curriculum (e.g. 6 children ran 5km in the regional cross-country championships, how far did they run altogether? 4 teams competed in a 20km waka ama race, how far did they travel each day?)
Problem 2 (Copymaster 3):
There are 256 rowers entered in the eights rowing champs at the Maadi Cup, not including the drivers (coxswains).
How many crews of eight rowers can be made?
Ask students to work out the answer in their head if they can and record their strategy on paper. Give the students an appropriate amount of thinking time. Then ask them to share their solutions with their learning partner. The following are possible responses:
Place value partitioning (chunking):
184 ÷ 8
I know that 160 ÷ 8 = 20. That is 20 crews.
There are 24 rowers left. 24 ÷ 8 =3
The answer is 20 + 3 = 23.
View how to do this using Video 4.
Factorisation (proportional adjustment):
Dividing by 8 is like dividing by 2 then 2 then 2 so half 184 is 92 and half 92 is 46 and divide by 2 again leaves me 23 so the answer is 23.
View how to do this using Video 5.
Rounding and compensating:
If there were 200 rowers, that would be 25 crews because 4 x 25 =100 so 8 x 25 = 200. 184 is 16 rowers less. That is two crews. So, the answer is 25 – 2 = 23.
As different strategies arise, ask the students to explain why they chose to solve the problem in that way. Accept all the strategies that are elicited at this stage, recording them to reflect upon later in the unit (perhaps in a modeling book). Watch for repeated subtraction or partial use of multiplication, such as:
10 x 8 = 80
80 + 80 = 160, 20 x 8 = 160
160 + 8 = 168, 168 + 8 = 176, 176 + 8 = 184
So 23 crews can be made
View how to do this using Video 6.
Ask students to reflect on the strategies that have been discussed in the session and evaluate which strategies that they personally need further work on, perhaps using thumb signals - thumbs up - confident and competent with the strategy, thumbs sideways - semi confident, thumbs down - not yet confident. Use this information to plan for your subsequent teaching from the exploring section outlined below.
To provide other related examples change the numbers in Problem Two (Copymaster 3), such as:
There are 212 rowers entered in the fours rowing champs at the Maadi Cup, not including the drivers (coxswains).
How many crews of four rowers can be made?
Exploring
Over the next two to three days, explore the following strategies, making explicit the strategy you are concentrating on as the teacher and the reason for using the selected strategy. For example, In the problem 7 x 29 tidy numbers would be a useful strategy as 29 is close to 30. When sharing, encourage students to also share and justify their strategy use. If you have a wide variety of strategies being used by different students, you might consider implementing a tuakana-teina approach, whereby students work collaboratively and learn from their peers.
The following questions are provided as examples for the promotion of the identified strategies. If the students are not secure with a strategy you may need to make up some of your own questions to address student needs.
The following questions can be used to elicit discussion about the strategy:
Place Value Partitioning (Multiplication)
Mani has $54, but he needs 7 times this amount to buy the new mountain bike he wants. How much money does the bike cost?
The place value strategy involves multiplying the ones, and tens separately then combining the partial products. This strategy applies the distributive property of multiplication, as 54 is distributed into 50 + 4. In the above problem the student might say the following:
I multiplied 7 x 50 and got 350, then I multiplied 7 x 4 and got 28. I added 350 and 28 to get 378.
The following questions can be used to elicit discussion about the strategy:
If the students do not seem to understand the partitioning concept, show the problems physically using place value materials, such as Place Value Blocks, BeaNZ and canisters, or Toy Money. Some students may find it useful to record and keep track of their thinking. An extension of the place value strategy involves the use of standard written form for multiplication.
Use the following questions for further practice if required:
Place value partitioning (division)
Pisi has an after-school job at the market, bagging pawpaw into bags of 6. If there are 864 pawpaw to be bagged, how many bags can he make?
The place value partitioning strategy for division involves ‘chunking’ known facts and subtracting them from the answer. Place value partitioning is the basis of the division written form or algorithm. In the case above, a student might think:
100 x 6 = 60 so 100 bags would be 600. 864 – 600 = 264. That leaves me with 264.
I can take 120 away from that, which is 20 x 6. That leaves 144. If I take another 120 pawpaw away I get 24, which is 4 lots of 6. So, I’ve taken away 100 lots, then 20 then 20, then 4… the answer is 144.
This thinking could be recorded as:
If the students do not seem to understand the partitioning concept, show the problems physically, e.g. using place value blocks. Students will find it useful to record and keep track of their thinking, and reduce memory load. An extension of the place value strategy involves the use of standard written form for division.
Use the following questions for further practice if required:
Rounding and Compensating (Multiplication)
The Southern Sting netball fans are going to Christchurch to watch a netball game against the Canterbury Tactix.
Each bus is full, with 48 people in it, and there are 9 buses.
How many Sting fans are heading to Christchurch?
The rounding and compensating strategy involves rounding a number in a question to make it easier to solve. In the above question 48 can be rounded to 50 (by adding 2). The problem then becomes 9 x 50 = 450. In order to compensate for the rounding, two lots of 9 people (18) must be subtracted from the ‘rounded’ equation.
If the students do not seem to understand the tidy numbers concept, use place value equipment or a large dotty array to show the problems physically. Some students may find it useful to record and keep track of their thinking. Recording might look like this:
View how to do this using Videos Seven and Eight.
Use the following questions for further practice if required, still using the same bus context:
Note that the problems posed here are using a tidying up strategy rather than tidying down. If one of the factors is just over a tidy number (such as 42) then standard place value partitioning tends to be a more useful strategy.
Rounding and compensating (Division)
Sarah uses nine bus tickets every week to travel around town. She wins 162 tickets in a radio competition. How many weeks will the tickets last her?
Rounding and compensating for division involves finding a number that is close to the dividend (starting amount) and working from that number to find an answer. For the question above, a student might say:
I know that 20 multiplied by 9 equals 180. 162 is 18 less than 180, that’s 2 x 9.
The tickets would last her 20 – 2 = 18 weeks.
If the students do not seem to understand the rounding and compensating concept, use place value materials, or a large dotty array, to represent the problems physically. Students may find it useful to record and keep track of their thinking, especially if they partially divide the dividend at first.
View how to do this using Video 7.
Use the same context of bus tickets to pose problems where rounding and compensating is a sensible strategy.
Proportional Adjustment (Factorisation)
At the Kapa Haka festival there are 32 schools with 25 students in each group, how many students are there altogether in the groups?
Proportional adjustment involves using knowledge of factors and multiples to create easier equations that have the same answer. Factors are proportionally adjusted to make one (or both) factors easier to work from. In the above problem the factors could be adjusted as follows:
Or, using doubling and halving:
The following questions can be used to elicit discussion about the strategy:
If the students do not seem to understand the proportional adjustment concept, use a large dotty array to show the problems physically. Some students may find it useful to record and keep track of their thinking.
View how to do this using Video 8.
Use the following questions for further practice if required:
Proportional Adjustment (Division)
A fishing company collects a total of 912 pipis over the course of six months.
How many pipis were collected each month?
In division, proportional adjustment involves changing both numbers in the equation by the same factor. Therefore, the numbers used to proportionally adjust the problem must be factors of both numbers in the equation (dividend and divisor). For example:
If I divide the 912 by 3 and 6 by 3, my equation becomes 304 ÷ 2 which has the same answer. Half of 304 is 152. So, each month 152 pipis were collected.
The following questions can be used to elicit discussion about the strategy:
If the students do not seem to understand the proportional adjustment concept, use equipment to show the problems physically. Some students may find it useful to record and keep track of their thinking.
Use the following questions for further practice if required. Consider how these questions can reflect the cultural diversity and learning interests of your class.
Factorisation (Multiplication and Division)
Stephanie has 492 extra marbles to share evenly amongst six of her friends. How many marbles will each person get?
The factorisation strategy involves using factors to simplify the problem. In this instance six can be factorised as 2 x 3. This means dividing by two, then three, has the same net effect as dividing by 6. Likewise, multiplying by two, then three, has the same net effect as multiplying by 6. In applying factorisation to the above problem, a student might think:
6 is the same as 2 x 3. So I halve 492, then third the result. If I divide 492 by 2 I get 246. 246 divided by 3 is 82. The answer is 82.
The following questions can be used to elicit discussion about the strategy:
If the students do not understand the factorisation concept, show the problems physically. Some students may find it useful to record and keep track of their thinking.
Use the following questions for further practice if required. Consider how these questions can reflect the cultural diversity and learning interests of your class.
Each day follow a similar lesson structure to the introductory session, with students sharing their solutions to the initial questions and discuss why these questions lend themselves to the strategy being explicitly taught. Conclude each session by having students make some statements about when this strategy would be useful and why (e.g. "Place value is useful when there is limited renaming required" or "Factorisation is useful when one of the factors is able to be renamed as a series of smaller factors"). It is important to record these key ideas as they will be used for reflection at the end of the unit.
Reflecting
As a conclusion to this unit of learning, give the students the following five problems in context to solve (Copymaster 4). Ask students to predict which strategy they think will be useful for each problem and why they think this is the most useful strategy before they solve the problem. After they have solved the problems, discuss the effectiveness of their selected strategies for the problems.
There may be problems for which two or more multiplication and division strategies are equally efficient. However, using additive strategies with these problems will not be efficient.
Problems for discussion (more than one strategy might be suitable for these)
Ask the students to create problems for a partner where one of the strategies covered in this unit is the most useful.
Conclude the unit by showing the students the questions asked in the initial session again and discuss whether they would solve them in a different way now, why or why not. Review the modeling book or record of statements or generalisations about the strategies and make any changes.
Family and whānau,
This week we have been exploring strategies for multiplying and dividing numbers. Ask your child to show you two different ways to solve each of these problems. They should be able to explain their thinking to you and show their thinking process with a diagram.
Printed from https://meaningfulmaths.nt.edu.au/mmws/nz/resource/multiplication-and-division-pick-n-mix-1 at 8:35pm on the 26th February 2024