In standard mathematical notation, when the brackets aren't there, the rule is that the multiplications EFFECTIVELY have brackets, i.e.
(4 x 4) + (4 x 4) + 4 - (4 x 4) = 20.
That's what other posters are calling the BODMAS rule, or something similar.
However, there is another notation called POLISH (as from Poland) where each sign is performed as it is encountered, reading left to right. Hence 320. But unless the person setting the question has clearly stated that it's in 'Polish notation', 320 is wrong and 20 is correct.