19 is correct if you follow the MOO. However it has been my experience in these kinds of puzzles that they often don't follow the MOO. Sometimes it's done just to frustrate the reader, sometimes it's out of ignorance.
I have a bunch of calculators and most of them follow the rules of precedent but I did find one that doesn't. Well I guess it's designed for financial calculations and precedent isn't that important. This one when I entered 4+3x5= it said 35. If I entered 3x5+4= then it said 19.