 Use strategies to add/subtract two digit numbers

Lesson

You might remember that there are several strategies to add and subtract numbers.  Maybe you had a favourite!

In this lesson, you'll use those same strategies to add and subtract two-digit numbers (numbers from $10$10 to $99$99).  Can you see the advantages of using each one?

Bridge to Tens

You can use the strategy of bridge to ten to add two-digit numbers.  Watch the video below to see why and how it works.   In what ways do you find this strategy useful?

Compensation

You can also use the compensation strategy to add and subtract two-digit numbers.  See how to use this strategy in the video below.   When might you use this strategy?

Number Line

Using a number line is another strategy that you can turn to when adding and subtracting two-digit numbers.  What times might using a number line be the best choice?

Examples

question 1

If $28+20=48$28+20=48, what is the answer to $28+18$28+18?

question 2

Use the number line to find the answer to $43+12$43+12.

1. First place the bigger of $43$43 and $12$12 on the number line.

2. Use the position of $43$43 to find the place of $43+12$43+12 on the number line.

question 3

What is $41-24$4124?

Outcomes

NA2-1

Use simple additive strategies with whole numbers and fractions

NA2-7

Generalise that whole numbers can be partitioned in many ways