Number bonds to 20
Maths · Key Stage 1 · The School
Bonds to 20 come from bonds to 10
You know the bonds that make 10: 1+9, 2+8, 3+7, 4+6, 5+5. Add a whole ten to one side. Now it is a bond to 20. 11+9=20. 12+8=20. 13+7=20. 14+6=20. 15+5=20. The ones digits stay the same pair. Just add a ten.
The full set of bonds to 20
20+0. 19+1. 18+2. 17+3. 16+4. 15+5. 14+6. 13+7. 12+8. 11+9. 10+10. Eleven pairs. Every one is a bond to 10, wearing an extra ten. They are not eleven separate facts. They are one idea, eleven times.
Checking subtraction with a bond
If 20 − 13 = 7, then 13 + 7 must equal 20. This is the same check used for smaller sums. Just bigger numbers now. Know your bonds to 20. See 13. Know its partner is 7, straight away.
Missing-number problems
14 + ⬜ = 20. Do not count on from 14. Recall the bond instead. 14's partner is 6. 4 and 6 make 10. The two tens are already there. So the missing number is 6. You recall it. You do not count it.
One missing-number problem, solved fast
A box holds 20 eggs. 16 have been used. How many are left? Write it as a bond: 16 + ⬜ = 20. Recall the bond to 20 for 16. Its partner is 4. 6 and 4 make 10. 10 and 10 make 20. 4 eggs are left. Worked out fast, not counted slowly.