Now we practise adding and subtracting with numbers up to 20. We use what we know about tens and ones to make this easier.
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Key Concepts
Adding with tens: 10 + 4 = 14. We just put the ones next to the ten.
Adding two numbers up to 20: Count on from the bigger number. For 8 + 5, start at 8 and count on 5 steps: 9, 10, 11, 12, 13. So 8 + 5 = 13.
Subtracting with tens: 15 - 5 = 10. Take away the ones first.
Subtracting from a teen number: For 14 - 6, think: "14 - 4 = 10, then 10 - 2 = 8." So 14 - 6 = 8.
Word problems: Read carefully, decide if you add or subtract, then solve.
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A farmer has 10 carrots. He grows 7 more. How many in total? 10 + 7 = 17.
There were 15 tomatoes. The farmer picked 5. How many are left? 15 - 5 = 10.
9 + 6 = ? Start at 9, count on 6: 10, 11, 12, 13, 14, 15. Answer: 15.
18 - 8 = ? 18 has 1 ten and 8 ones. Remove 8 ones: 10 is left.
A bag has 12 potatoes and 4 onions. How many vegetables are in the bag? 12 + 4 = 16.
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Common mistakes
- When adding, always start counting from the bigger number.
- In subtraction, take ones away first before dealing with tens.
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Summary
We can add and subtract numbers up to 20 by using tens and ones, counting on, or breaking numbers into parts. Word problems tell us which operation to use.