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pietersbrendan
9+9=18 -1     hope this helped 
Mackenzie9980
Ok, so an easy way that I have used to help me with adding to 9's is imagine that the 9 is 10.  if you added 8 + 10 it would be 18.  So then you just subtract 1 from 18 which would be the equivalent of 8 + 9.  So 8 + 9 is 17.