Adding and subtracting within 20: what children learn
In the game, children balance the scales: they choose the number that makes both sides equal, such as 8 + ? = 15 or 9 + 6 = 10 + ?, which shows that = means “the same as”. The Quick quiz mixes four kinds of question: addition (8 + 5 = ?), subtraction (14 − 6 = ?), a missing first number (? + 4 = 11) and a calculation written the 'other way round' (7 = ? − 9). Seeing the equals sign in different positions teaches the important idea that = means 'is the same as', not 'here comes the answer'.
Supports part of Common Core 1.OA.C.6. This focused game does not cover the entire standard.
Common Core connection: 1.OA.C.6. This is focused practice for part of the standard.
How to play
Make both sides of the scales equal. Tap the weight that fills the gap, or press 1 to 4. Each game has 8 questions and no timer. If a weight doesn't balance, the scales tip towards the heavier side and you get one more try. You get a point for each one balanced first time. For a quicker go, choose Quick quiz: 10 multiple-choice questions in 120 seconds (choose Relaxed for more time).
Step by step
- Add by counting on from the bigger number. For 3 + 9, start at 9 and count on 3: 10, 11, 12. Starting from the bigger number is quicker and less error-prone.
- Bridge through 10. For 8 + 5, split the 5 into 2 and 3. 8 + 2 = 10, then 10 + 3 = 13. This uses number bonds to 10.
- Subtract by counting back or finding the difference. For 14 − 6 you can count back 6 from 14, or count up from 6 to 14 (that is 8 steps). Both give 8.
- Use the inverse for missing numbers. For ? + 4 = 11, ask what you add 4 to in order to make 11. That is the same as 11 − 4 = 7. For 7 = ? − 9, ask what number take away 9 leaves 7: that is 7 + 9 = 16.
Worked examples
Example 1: 8 + 5 = ?
Working: 8 + 2 makes 10, and 3 more makes 13.
Answer: 13
Example 2: 15 − 7 = ?
Working: 15 − 5 = 10, then take away 2 more to get 8.
Answer: 8
Example 3: 7 = ? − 9
Working: The missing number take away 9 equals 7. Adding is the inverse, so the missing number is 7 + 9 = 16. Check: 16 − 9 = 7.
Answer: 16
Common mistakes and how to fix them
- Thinking 7 = ? − 9 is 'back to front' and cannot be solved. Read the equals sign as 'is the same as'. Use a balance or see-saw picture: both sides must be worth the same.
- Taking the smaller number from the bigger one regardless of order. In this game all subtraction answers are 0 or more, but the order still matters. Use objects to show that 3 − 8 is not the same as 8 − 3.
- Counting the starting number when counting on (8 + 3 counted as 8, 9, 10). Say the start number, then count the jumps: 'eight… nine, ten, eleven'.
Activities to try at home
- Play shop with toys priced up to 20¢ and work out totals and change.
- Roll two dice (or dice with up to 10) and add the scores. Then take the smaller from the bigger.
- Write a missing number question on a sticky note each morning and see who solves it first.
Ideas for the classroom
- Use part-whole models: a circle for the whole and two circles for the parts. Hide one part to create missing number problems.
- Use a bead string or number line to 20 to show counting on and counting back.
- Ask students to write a 'fact family' for a calculation: 9 + 7 = 16, 7 + 9 = 16, 16 − 7 = 9, 16 − 9 = 7.
Free printable game and worksheet
The free printable starts with a game sheet, such as a color-by-answer picture, an answer maze or a secret-code joke, followed by a worksheet. Both use the same checked questions as the online game, and the answers are on the last page. Print it, or save it as a PDF from your browser’s print menu.
Open the free printable game and worksheet
Frequently asked questions
What does 'within 20' mean?
All the numbers in the calculation, including the answer, are 20 or less. The game never goes above 20.
How should I choose this game for my learner?
Supports part of Common Core 1.OA.C.6. This focused game does not cover the entire standard.
Should my child use fingers?
Fingers are fine to begin with. Over time, encourage strategies such as number bonds and bridging 10 so that facts become quick to recall.
How should I choose this game for my learner?
Supports part of Common Core 1.OA.C.6. This focused game does not cover the entire standard.