2.6 Multiplication Written Strategies

Unit Objectives

• We are learning to use written strategies to multiply larger numbers.

Success Criteria

• I can use an area model and extended multiplication to multiply a 1-digit number by a 2-digit number.

• I can use contracted (compact) multiplication to solve multiplication problems.

• I can choose an efficient strategy — mental or written — to solve multiplication problems, including 2-digit by 2-digit.

Learning Objective

• To multiply a 2-digit number by a 1-digit number using an area model.

Success Criteria

• I can split a 2-digit number into tens and ones.

• I can model a multiplication problem using base-10 blocks or an area model.

• I can find the total by adding the parts together.

Work together to solve.

The P&C orders 18 boxes of muffins for the Welcome BBQ, with 6 muffins per box. How many muffins in total?

3 x 24 =

 - Base-10 blocks -

Learning Objective

• To record extended multiplication as a written method.

Success Criteria

• I can write a multiplication problem in the extended (expanded) written format.

• I can multiply the ones first, then the tens, recording each step.

• I can add my partial answers to find the total.

On your whiteboard, solve using the area model.

Four classes each plant 23 seedlings in the veggie garden. How many seedlings altogether?

Extended Written Algorithm (I Do)

Oxford Interactive Tool

Repeat with a student example.

Extended Written Algorithm (We Do)

Work together to solve. Take turns telling each other what to do.

Mark as a class.

Learning Objective

• To use shorter way to record multiplication called contracted multiplication.

Success Criteria

• I can set out a contracted multiplication problem correctly.

• I can multiply the ones and trade correctly into the tens column.

• I can compare extended and contracted multiplication and explain how they relate.

On your whiteboard, solve using the area model or extended algorithm.

The library orders 5 new bookshelves, each holding 27 books. How many books in total?

Oxford Interactive Tool

Learning Objective

• To multiply a 2-digit number by a 2-digit number using the grid method.

Success Criteria

• I can split both numbers in a 2-digit × 2-digit problem by place value.

• I can use a grid to organise and add the four partial products.

When would you multiply a 2-digit number by a 2-digit number?

-turn and talk-

Learning Objective

• Understand how the associative property can help with mentally doing harder multiplications faster. 

Success Criteria

• I can explain what the associative property means.

• I can recall multiplication facts to identify factors.

• I can group friendly factors to help multiply.

🤔 ⏱️

How could you solve 5 x 18 mentally?

What if we could turn it into 10 × 9 instead?

This is the lesson goal:
Use the associative property to turn hard multiplications into easy ones.

Let's look at the factor 18.

Think of it as a product now, what factors make 18?
Write them on your whiteboard.
Hint: What times tables make 18?

We will use the factors 2 and 9.

2 x 9 = 18

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Why is 2 x 9 useful here?

Because we can use the associative property and the factor 2 first to make multipling the original equation 5 x 18 easier. 

Let's solve:

We: 5 x 22

You: 5 x 24

What about?

We: 6 x 25

Partners: 4 x 35

You: 8 x 15

Hint: Use the factors of the first number to make it easier.

Let's play.

5 x 18 = 

5 x (2 x 9) = (5 x 2) x 9

= 10 x 9

= 90
Why doesn't regrouping the brackets change the answer?

Learning Objective

• Identify the different strategies learned to multiply 2-digit numbers.

Success Criteria

• I can use a strategy of my choice to answer multiplication problems.

What are the different strategies you have learned to multiply 2-digit numbers?
-turn and talk-

Solve the following questions using the strategy.