Adding and subtracting within 20: what children learn
In Year 1, children learn to add and subtract one-digit and two-digit numbers to 20, including 0. They read and write calculations using the +, − and = signs, and solve simple problems including missing number questions such as 7 = ? − 9.
This game 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'.
This topic covers the Year 1 statutory requirement in the National Curriculum in England (Number – addition and subtraction): pupils should be taught to “add and subtract one-digit and two-digit numbers to 20, including 0; solve missing number problems such as 7 = ? − 9”. Year 1 pupils are usually aged 5–6.
For families following US standards, the closest Common Core standard is 1.OA.C.6, which is taught in Grade 1.
How to play
Answer 10 questions before the clock runs out. Tap an answer, or press 1 to 4. Each game has 10 multiple-choice questions and a 120-second timer. A wrong answer shows the correct one before moving on. You earn up to three stars depending on your score.
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 Year 1 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 20p 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 pupils to write a 'fact family' for a calculation: 9 + 7 = 16, 7 + 9 = 16, 16 − 7 = 9, 16 − 9 = 7.
Printable worksheet
The printable worksheet uses the same question generator as the game, so it practises exactly the same skills. It comes with a separate answer sheet on the second page. Print it, or save it as a PDF from your browser's print menu.
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.
Why are there questions like 7 = ? − 9?
The Year 1 programme of study names this exact type of missing number problem. It helps children understand the equals sign properly, which matters a lot for algebra later.
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 is this different from the Year 2 facts game?
The maths is similar. Year 2 focuses on fluent recall against the clock, while this Year 1 game is multiple choice so children can concentrate on the method.