Number (add/sub)
Hong Kong
Stage 1
Addition fact families (a+b=c, c-b=a)
Lesson
A fact family describes a relationship between 3 numbers. The relationship could be through addition or subtraction.
This video shows how we can use fact families to help us answer questions.
Worked Examples
Question 1
We are interested in the fact family of $1$1,$8$8 and $9$9.
 |
$+$+ |  |
$=$= |  |
| $8$8 | $+$+ | $1$1 | $=$= | $9$9 |
What is $1+8$1+8?
What is $9-8$9−8?
What is $9-1$9−1?
Question 2
We are interested in the fact family of $2$2, $4$4 and $6$6:
 |
$-$− |  |
$=$= |  |
| $6$6 | $-$− | $4$4 | $=$= | $2$2 |
What is $2+4$2+4?
What is $6-2$6−2?
What is $4+2$4+2?
Question 3
We are interested in the fact family of $2$2,$4$4 and $6$6:
 |
$+$+ |  |
$=$= | ? |
| $2$2 | $+$+ | $4$4 | $=$= | ? |
What is $2+4$2+4?
What is $4+2$4+2?
What is $6-4$6−4?
When looking at the answer to $6-2$6−2, is the answer going to be less than $10$10?
Yes
ANo
BWhat is $6-2$6−2?