Explain when it is efficient to use factorising or long multiplication to multiply by 2-digits
Lesson details
Learning outcome
I can explain when it is efficient to use factorising or long multiplication to multiply by 2-digit numbers.
Key learning points
- Multiplication can be simplified by factorising one of the factors.
- If the multiplication will involve regrouping then a written method may be more efficient.
- If the multiplication will not involve regrouping then a mental or informal method may be more efficient.
Keywords
Associative law - The associative law states that it doesn't matter how you group or pair values (i.e. which we calculate first), the result is still the same. It applies for addition and multiplication.
Efficient / efficiently - Working efficiently means finding a way to solve a problem quickly whilst also maintaining accuracy.
Common misconception
Pupils can 'default' to using a written algorithm when learned regardless of the values of the calculation.
Consider using confident mathematicians in a race to show efficiency of different methods. Encourage pupils to 'interrogate' the values of the calculation in order to identify more efficient approaches.
Teacher tip
Part of this lesson involves identification of prime numbers. Consider how confident your pupils are when reasoning about whether 2-digit numbers are prime or not and how you may support them to do this.
Licence
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.Select the prime numbers.
Q2.Select the factor pairs of 30
Q3.Look at the numbers below and select the composite numbers.
Q4.Select the equivalent expressions which would make this equation correct. 36 × 24 =
Q5.Find the expressions that are equivalent to 35 × 4 × 8
Q6.Look at the equation. Find the missing number. 42 × 49 = 42 × 7 ×
Assessment exit quiz
6 Questions
Q1.Select equivalent expressions to 21 × 30
Q2.Which method is more efficient to solve 47 × 16 = ?
Q3.Which method is more efficient to solve 132 × 15 = ?
Q4.Which factorised example would be most efficient to solve 36 × 12?
Q5.Use an efficient method to solve: The Olympic event is selling tickets. Each ticket costs £32 and 29 tickets are sold in the first five minutes. What is the total amount spent? £
Q6.Use an efficient method to solve: 407 × 15 =
To help you plan your 6 maths lesson on: Explain when it is efficient to use factorising or long multiplication to multiply by 2-digits, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your 6 maths lesson on: Explain when it is efficient to use factorising or long multiplication to multiply by 2-digits, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
We use learning cycles to break down learning into key concepts or ideas linked to the learning outcome. Each learning cycle features explanations with checks for understanding and practice tasks with feedback. All of this is found in our slide decks, ready for you to download and edit. The practice tasks are also available as printable worksheets and some lessons have additional materials with extra material you might need for teaching the lesson.
The assessment exit quiz will test your pupils' understanding of the key learning points.
Our video is a tool for planning, showing how other teachers might teach the lesson, offering helpful tips, modelled explanations and inspiration for your own delivery in the classroom. Plus, you can set it as homework or revision for pupils and keep their learning on track by sharing an online pupil version of this lesson.
Explore more key stage 2 maths lessons from the Multiplying and dividing by 2-digit numbers unit, dive into the full primary maths curriculum, or learn more about lesson planning.