@Dustin57 there is no need to use the SUM function for an arithmetical formula in XL. As John says, just go for =A1*0.9; in this case, the SUM function is actually doing nothing in your formula.
If I'd got hear earlier, I would have suggested:
A1: 100
B1: =(A1-(A1*10%)) - a few characters more, but a completely readable / understandable formula.