Level 3

Patterns in addition and subtraction I

Lesson

Finding the pattern

When we know the answer to one addition (or subtraction) number problem, we can use that to solve other problems. By noticing which digit has changed, such as the tens, we can make a change to our total of the same amount. Do you see any pattern here?

$16+12=28$16+12=28

$26+12=38$26+12=38

$36+12=48$36+12=48

For our first video, let's look at addition problems, in the video below. Don't worry though, you can also look at how we find patterns in subtraction number problems. The first video of that chapter will show you how.

regrouping

Sometimes our units add up to 10 or more, or we haven't enough units to subtract. In that case, we need to think about how we can regroup, to solve our problem. This time, our video looks at subtraction number problems, but you can also see how we tackle addition problems with regrouping, by looking at the second video in this chapter.

Remember!

Whether we are solving addition or subtraction problems, we can still look for patterns to make it easier to solve further problems. Remembering the place value of our digits is important, and can help us to identify patterns.

Worked Examples

Question 1

Use what you know about subtracting to find:

$38-27=\editable{}$38−27=
$58-27=\editable{}$58−27=
$68-27=\editable{}$68−27=
$98-27=\editable{}$98−27=
What is the pattern? Choose the correct answer:
When we subtract $27$27 from any two digit number, the answer will always end in $8$8.
A
The tens digit never changes when we subtract $27$27 from any two digit number.
B
When we subtract $27$27 from a number that ends in $8$8, the answer will always end in $1$1.
C
There is no pattern.
D

Question 2

We know that $38-25=13$38−25=13.

Use this to find:

$\editable{}-25=23$−25=23
$\editable{}-25=33$−25=33
$\editable{}-25=43$−25=43
$\editable{}-25=73$−25=73
What is the pattern? Choose the correct answer:
A two digit number minus $25$25 always has the same answer.
A
When we subtract $25$25 to a two-digit number that ends in $8$8, we get a two-digit number that ends in $3$3.
B
The tens digit never changes when we take $25$25 away from any number.
C
There is no pattern.
D

Question 3

$54-37=17$54−37=17. Use this to find:

$64-37=\editable{}$64−37=
$74-37=\editable{}$74−37=
$84-37=\editable{}$84−37=
$94-37=\editable{}$94−37=

Outcomes

NA3-3

Know counting sequences for whole numbers.

NA3-6

Record and interpret additive and simple multiplicative strategies, using words, diagrams, and symbols, with an understanding of equality