Basically your final mark is broken into two: your external and internal mark. The external is the final exam that you sit in October/November which is worth 50% of the final mark. The internal is the other 50% of the total which consists of your trial plus your school assessments.
Your trial that you said was worth 40%, is actually 40% of your internal, so really you just halve it to work out the amount as a percentage of your final, which is 20%. Therefore your trial is worth about one fifth of your final mark. If you've screwed up your trials, it's not the end. If you work hard and do well in your finals, you can still get a decent mark. Hope that helps.