Navigation
Students master adding and subtracting numbers up to 20 using various strategies.
Imagine you have a treasure chest with 9 gold coins, and you find 4 more hidden in the sand—how can you quickly find your new total without counting from one every single time?
When we add, we don't always have to start at 1. We can use a trick called counting on. To do this, you put the bigger number in your head and 'count up' the smaller number. For example, if you have , you don't count '1, 2, 3...'. Instead, you start at 9 and just say the next three numbers: '10, 11, 12!' This saves time and helps you handle bigger numbers up to 20 easily. It is like taking a shortcut on a number line!
Let's solve using the counting on strategy: 1. Identify the bigger number: . 2. Put in your head. 3. Count up 5 more fingers: . 4. The answer is !
Quick Check
If you are adding , which number should you put in your head first to use the 'counting on' strategy?
Answer
You should put 12 in your head because it is the bigger number.
Addition and subtraction are opposites, just like 'up' and 'down.' They belong to the same fact family. A fact family is a group of three numbers that work together. If you know that , you already know three other facts! You know , and you also know the subtraction facts: and . Understanding this relationship helps you solve subtraction problems by thinking about addition.
If you know the numbers and are in a family, you can write four equations: 1. 2. 3. 4.
Quick Check
If you know that , what is ?
Answer
2
Just like we count forward to add, we can count back to subtract. To solve , start at 15 and count backward three steps: '14, 13, 12.' Another way is to 'think addition.' To solve , ask yourself: '18 plus what equals 20?' Since , then . Using these strategies makes working with numbers up to 20 feel like a game!
Solve by thinking about addition: 1. Ask: 2. Count on from 9 until you reach 17: . 3. You counted numbers. 4. So, .
What is using the counting on strategy?
Which equation belongs in the same fact family as ?
True or False: When you subtract, the answer (difference) is always larger than the starting number.
Review Tomorrow
In 24 hours, try to remember the three numbers in the '15 family' (10, 5, and 15) and say all four equations out loud.
Practice Activity
Find 20 small items (like cereal or buttons). Practice 'counting on' by hiding some in a cup and adding more to the pile!