Given that Mathematics Extension 1 is treated as assumed knowledge rather than a prerequisite (the exception being USyd, although you can still study this degree there having taken and performed at a sufficiently high level in Mathematics Advanced), you can still study a civil engineering degree despite not having taken this level of maths. With that being said, you will likely need to catch up on concepts taught in Mathematics Extension 1 in order to maximise your performance as academics will assume that you are already familiar with them. There may be two ways of doing so:
- See if you can familiarise yourself with the main topics from Mathematics Extension 1 that will be most relevant to your studies of civil engineering between now and when you start university.
- Complete a bridging course with a focus on Mathematics Extension 1 so that this lack of knowledge is addressed in an effective manner, placing you in an appropriate position as you commence your studies of engineering.
In terms of a business degree, Mathematics Advanced is generally considered sufficient for most specialisations, including the more numerical/mathematical ones (such as finance, accounting, economics and business analytics), whereas it will be more than sufficient for non-mathematical majors which involve little to no maths (such as management and human resources). Additionally, an HSC result in the mid-band 5 for Mathematics Advanced range suggests that you have the ability to perform well in a business degree, especially if you choose one of the more mathematical majors.
Regarding whether to study a double degree, this depends on your approach to addressing the concern of assumed knowledge and whether you strongly believe that you are capable of effectively managing the workload imposed by two degrees. You definitely seem capable of studying and performing favourably in a Bachelor of Business/Commerce.
I hope this helps!
