iCloud Numbers can't handle decimals

Why is it that when I type =SUM(15+15+15) I get 45 but if I type =SUM(15.00+15.00+15.00) I get a red triangle and a message that says "The formula contains a syntax error"? I'm trying to make an accurate budget and I really want those decimals.

The same thing happens is I type =(15+15+15) versus =(15.00+15.00+15.00).

To make things more weird, this only happens when I type the numbers, not when I use cell references, as in =SUM(B2+C2+C3). If any or all of those cells contains decimals I still get a result.