Number (add/sub)

Hong Kong

Stage 1

Lesson

Here we will use patterns that we find in addition problems to help us add two-digit and single-digit numbers.

Sometimes when we add we might need to regroup. Here is a regrouping example. Suppose we have twelve unit blocks, and then we regroup this to be one ten and two ones. We often use blocks to help us visualise this.

Let's have a look at an example, can you see a pattern in the both the sums and the answers?

$16+9=25$16+9=25

$26+9=35$26+9=35

$36+9=45$36+9=45

Let's have a closer look in this video:

$19+2=21$19+2=21. Use this to find:

$39+2=\editable{}$39+2=

$49+2=\editable{}$49+2=

$59+2=\editable{}$59+2=

$79+2=\editable{}$79+2=

Let's try and get to the answer we want!

$39+\editable{}=42$39+=42

$49+\editable{}=52$49+=52

$59+\editable{}=62$59+=62

$19+\editable{}=22$19+=22

Which of these is the most correct?

$3$3+$9$9 = $13$13

AThe answer to the last four questions was always $3$3 because the answer to every mathematics question is $3$3.

BThe answer to the last four questions was always $3$3 because only the tens digit changed.

CWhen a one-digit number is added to a two-digit number, the tens digit always changes.

D

Choose the right answer.

$29+3$29+3 will be in the:

20s

A30s

B