Addition and Subtraction Facts to 18

Strand: Number
Outcome: 10

Step 1: Identify Outcomes to Address

Guiding Questions

What do I want my students to learn?
What can my students currently understand and do?
What do I want my students to understand and be able to do, based on the Big Ideas and specific outcomes in the program of studies?

See Sequence of Outcomes from the Program of Studies

Strand: Number

Grade 1

Grade 2

Grade 3

Specific Outcomes

10.

Describe and use mental mathematics strategies (memorization not intended), such as:

counting on and counting back
making 10
using doubles
thinking addition for subtraction

for basic addition facts and related subtraction facts to 18.

Specific Outcomes

10.

Apply mental mathematics strategies, such as:

using doubles
making 10
one more, one less
two more, two less
building on a known double
thinking addition for subtraction

for basic addition facts and related subtraction facts to 18.

Specific Outcomes

10.

Apply mental mathematics strategies and number properties, such as:

using doubles
making 10
using the commutative property
using the property of zero
thinking addition for subtraction

for basic addition facts and related subtraction facts to 18.

Big Ideas

Numbers are related to each other in numerous ways. For example, 8 is 2 more than 6, 2 less than 10, 1 more than 7, 1 less than 9, double 4, composed of 3 + 5 or 1 + 7, and can be recognized in several dot arrangements, such as two sets of 4 or a set of 5 and a set of three. Eight is also related to 18, 28, 38 and many other numbers.
The order of adding addends does not change the sum. This is the commutative property of addition. Knowing this reduces the number of addition facts a person needs to learn in half. For example, if you know 3 + 5 = 8, then you also know 5 + 3 = 8. The commutative property does not apply to subtraction.
Many of the basic fact sums and differences can be generated by using a small number of known facts, such as doubles and combinations that make ten, freeing up your memory and reducing effort. It is much easier to access information that is interconnected to other information than to recall isolated, discreet facts.
The relationship to 5 and 10 of other numbers is very useful, as these two numbers form benchmarks during mental calculations of math facts.
Addition and subtraction are inverse operations, so you can do subtraction by thinking addition. This reduces the number of basic facts students must learn. Students generally find addition facts much easier to learn than subtraction.

This is an even more powerful outcome than just learning the facts with these strategies. If students can apply the mental math strategies to problems with larger numbers, they will have demonstrated their growing number sense and have a good foundation for addition and subtraction using larger numbers.