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Nrich submission: Digit Addition
We agree with Jonas but we have a different way to explain it. He said that adding 9 is the same as adding 10 and subtracting 1.
Our working out:
1+9=10 1+0=1 2+9=11 1+1=2 3+9=12 1+2=3 4+9=13 1+3=4 5+9=14 1+4=5 6+9=15 1+5=6 7+9=16 1+6=7 8+9=17 1+7=8 9+9=18 1+8=9
So we conclude that every time you add 9 to a number from 1 to 9 and then add your answers digits together, you will always get the same number you started with. If you wrote 1 to 9 you will get the same order back.
Here’s proof that you can’t go over 10.
23+9=32 3+2=5
By Emma and Ruby. 
Yewdale Primary School, Year 4 Tolkien class