If it's silly mistakes and panicking the only solution that comes to mind is timed papers and proper exam technique. In other words, even more time devoted to maths. And some courses have Maths as assumed knowledge although you can take a bridging course before Uni starts.
Cheap or not, HSC results haven't even come out yet. Quality remains questionable unless you went to a high ranked school and had the internals to back it up lol.
To find the mass of fuel burnt, you'll need the amount of moles i.e. n = g/molar mass
To find the moles, given molar heat of combustion (2800kJ), divide the heat generated by 2800.
Then multiply the moles by molar mass to get the mass.