Number Bonds to 20
Number bonds to 20 extend the pairs-that-combine-to-a-fixed-total idea from 10 up to 20, covering pairs such as 12 + 8 and 15 + 5, with a particular focus on pairs involving a teen number since these make mental addition and subtraction within 20 fast.
Method
- Confirm bonds to 10 are already secure and instantly recalled - this whole topic is built on that fact, not a fresh memorisation task.
- Use a twenty-frame (two ten-frames stacked or side by side) so a full ten is always visibly boxed off: fill it with counters and ask what is missing, exactly as with the ten-frame, but now the first ten fills up completely before the second ten starts.
- Teach the link explicitly with a part-part-whole bar split into 'one whole ten' plus 'some extra ones': since every number from 11 to 19 is one ten plus some ones (e.g. 14 is 1 ten and 4 ones), finding what completes it to 20 only means finding what completes the ones digit to 10 - the ten is already there.
- Practise saying this link out loud with several examples until it becomes automatic: '13 needs 7, because 3 and 7 make 10'; '17 needs 3, because 7 and 3 make 10'; '11 needs 9, because 1 and 9 make 10'.
- Treat 10 + 10 and 0 + 20 as the two special cases outside this pattern, and make sure the child meets both, since children who only ever practise the teen-number pattern can be caught out by 'what goes with 10 to make 20?'.
- Move to the abstract stage - missing-number sentences such as 20 = 16 + ? or ? + 12 = 20 - once the picture and the ones-to-ten link are secure, varying where the gap sits in the sentence.
- Watch for children who try to count on from the smaller number one at a time for every question: this works but is slow and error-prone past about 5 counts, so if you see finger-counting from 1 for something like 'what goes with 12 to make 20', go back to the ones-digit link rather than letting the habit bed in.
Worked example
What number goes with 15 to make a bond to 20?
- Split 15 into a whole ten and some ones: 15 = 1 ten and 5 ones.
- The ten is already there, so only the 5 ones need to be completed to 10.
- Recall the bond to 10 that goes with 5: 5 + 5 = 10.
- So the number that goes with 15 to make 20 is 5 (check: 15 + 5 = 20).
Practice questions
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Q1What number goes with 14 to make a bond to 20?Show answer
Answer: 6 (14 + 6 = 20)
Q220 = 11 + ?. Write the missing number.Show answer
Answer: 9
Q3What number goes with 10 to make a bond to 20?Show answer
Answer: 10
Q4Write the missing number: ? + 17 = 20.Show answer
Answer: 3
Q5A jar holds 20 marbles when full. Elowen has put in 18 marbles so far. How many more marbles does she need to fill the jar?Show answer
Answer: 2 marbles (18 + 2 = 20)
Q6Circle the pair that does NOT make a bond to 20: (12 and 8) (9 and 11) (13 and 8)Show answer
Answer: 13 and 8 (13 + 8 = 21, not 20)
Q7What number goes with 19 to make a bond to 20?Show answer
Answer: 1
Exam-style questions
Written in the style of a KS1 Maths exam paper, with a full mark scheme.
Solomon says that the number that goes with 16 to make 20 is the same as the number that goes with 6 to make 10. Is Solomon correct? Explain why, using the fact that 16 is 1 ten and 6 ones.
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Poppy has 20 beads to make a bracelet. She has threaded 8 blue beads. She wants the rest to be red. How many red beads does she need? Show your working.
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Fill in the two missing numbers so that both number sentences are bonds to 20: ? + 15 = 20 and 20 - 9 = ?.
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Free printable worksheet
Want more practice on paper? Download the number bonds to 20 worksheet pack - 7 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
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