FWIW, your DD's way sounds as though it could be just as quick as the way you think she should do it, to me! I wouldn't sweat it, other than maybe to share how you did something in an "isn't it interesting that there are different ways to get the same answer" way. My DS will sometimes answer from one calculation and then point out another way, adding "so it makes sense", and I think that's to be encouraged. If one way really is faster than another, I expect it'll be the one adopted in the end....
