Addition within 20 - Primary 1

Let's talk about what students will learn in this chapter on addition within 20:

• adding two 1 digit numbers
• adding 1 digit and 2 digit numbers
• Word problems

Adding two 1 digit numbers

Here, you'll learn how to add two 1 digit number to form a number between 10 and 20. There are many ways to add 2 numbers together.

In Singapore Math, the method most students learn to add two numbers together involve splitting the  numbers up. Hence, students will need to make use of their knowledge of  number bonds learnt in earlier chapter. 

Let's use an example to illustrate how students learn addition (of small numbers between 10 - 20) in Singapore. To find 8 + 7, we know that this would give a number greater than 10. This is because 8 + 2  gives 10, but instead of having 2, we have 7. Since 7 is greater than 2, the answer will definitely be greater than 10. We are checking that the answer is greater than 10, as this method that we will be learning is for addition of 1 digit numbers with a sum between 10 and 20. 

Now that we know that this method is applicable, let's go through how to add 8 to 7 step-by-step.

Step 1: Look at number 8. Determine what number makes ___ + 8 makes 10. We know that it's 2.

Step 2: Split 7 into 2 +  ___. We know that 7 can be split into 2 + 5.

Step 3: Since 7 can be split into 2 + 5, and 2 + 8 makes 10, we know that we have 5 left. Hence 8 + 7 = 15.

How to do 8 + 7 using the Singapore Maths method

Adding two 1 digit and 2 digit numbers

In this chapter, students learn to add a 1 digit and 2 digit numbers together. The concept of number bonds is used here as well. 

Let's use an example to illustrate how students learn addition of a 1 digit number to a 2 digit number to get a number within 20. Again, we'll be making use of number bonds. 

In this example, let's look at adding 3 to 14. To do so, we'll split 14 into 10 and 4. Then we'll add 4 to 3 before adding 10. This gives 17. 

Word problems 

Now that we have learnt addition within 20, it's time to apply this to word problems. For word problem involving addition, you'll usually see key words such as "altogether", and " in all".