As we have friends coming round soon for New Year (and my wife seems to think it rude if I sit on MR while people are visiting
), I'll tell you the answer and officially declare it SS's turn.
If you draw a single straight line from two points of the second '+' sign (from North to West as it were), it makes that look like a 4. So the sum becomes 5+545=550.
This YouTube video shows it (and no, that's not me, I can spell 'impossible'):