In the chapter on subtraction within 20, this is what students learn:
- subtraction within 20
- word problems
Subtracting a single digit number from a 2- digit number
In Singapore Maths, a lot of basic addition and subtraction are done using the concept of number bonds. Using number bonds is a very good way of helping students have a good grasp of numbers - how numbers can be combined or split.
Let's look at a few example of how to do subtraction within 20:
Example 1:
19 - 7 =
To subtract 7 from 19, we will first split 19 into 10 +9.
Next, we take 9 - 7 to get 2.
As we are left with 10, we'll take 10 + 2 to get 12.
Hence, 19 - 7 = 12.
Example 2:
13 - 8 =
To subtract 8 from 13, we'll first split 13 into 10 + 3.
Since 3 is smaller than 8, we'll subtract 8 from 10 instead. 10 - 8 gives 2.
We'll add 2 back to 3 to get 5.
Hence, 13- 8 = 5.
Example 3:
17- 15 =
To subtract 15 from 17, we'll split both 15 to 10 and 5, and 17 to 10 and 7.
10- 10 = 0
7 - 5 = 2
Hence, 17 - 15 = 2.
At the start, doing such subtraction seems long and tedious, but once students internalise this, they'll be able to do this mentally and quickly.
Word problems involving subtraction
Once students have learnt subtraction, the next thing is to apply them to word problems. Key words such as "left" suggest subtraction.
