Look at this number square, which shows all the bonds you should know by heart up to 20.
Example 1
Add 6 + 9
Answer
You should know this by heart, but the number square shows that it is 15.
Example 2
Subtract 15 - 9
Answer
You should know this by heart, but the number square shows that it is 6.
When you subtract, the starting number is inside the square.
Example 3
Add 12 + 5
Answer
You should know that 2 + 5 = 7,
so 12 + 5 = 17
Example 4
Subtract 16 - 12
Answer
You should know that 6 - 2 = 4,
so 16 - 12 = 4 as well.
