Okay. This is a little tricky because your question isn't quite as clear as I would like it to be, but I can work with it nevertheless. I am going to assume that you are asking how many tenths you would get when you add 0.12 and 0.9, which are the equivalent to 12 tenths and 9 tenths. The answer to this would be 0.21, or 21 tenths, or if you want to get really technical and use the absolute correct term, 21 hundredths. If this answer was not clear, please ask! Happy home-working!
