What does make a ten mean in 17-8?

Answer 1

25

Answer 2

This is one of the strategies suggested in the Common Core State Standards for Mathematics. With practice, this strategy can theoretically help younger students gain fluency when subtracting within 20.

I am not a math theorist, so it's mystifying to me how adding an extra step that one is supposed to forget later when the process is memorized could be helpful.

17 - 8 = 17 - 7 - 1 = 10 - 1 = 9