The assessment doesn't really matter to be honest. Obviously if you don't answer any of the questions right then you might be in trouble - and in this case I'd recommend you to go back to primary school, REALLY.
But from what I've heard they only look at the assessment if they have got any final doubts about giving you an offer and most of the people that are invited to attend the applicants day will have an offer so that should've been an automated message.